My girlfriend told me that she thought I'd look better if I stopped doing heavy lifting (squats, deadlifts, presses, etc...) and did more cardio. She said it would improve my health to lose some of my "bulk". I stand 6'1 and weigh 230lbs. I'm concerned that other men will lose respect for me. Additionally, my current size instills fear in other men. What happens if they stop fearing me? How does my size and situation compare to yours? What would you do if you were in my shoes?
You have posed a difficult question that hits quite close to home, and fortunately yields a simple answer I will deal first with the philosophical and then move on to the practical.
The simple answer is that decisions regarding your physique (and your dress) are yours alone to make. This is the epitome of what it means to be a man: to be fully independent, to require no outside counsel, to make decisions alone and bear alone the consequences. Only you are in the position to make the decision regarding what is right for you: only you can truly understand the internal utility generated or destroyed by different choices in physique. When your Biggie began his now two-year relationship with his present girlfriend, he declared on the second date--or perhaps the first?--that the Biggie she saw was the Biggie she would get. Indeed, your Biggie has had experience changing his physique to suit the whims of others: it is bound to fail. Externally-motivated changes in physique are doomed to failure, and in the event that such a change succeeds, it can lead to psychological destruction. The desire to maintain or change a physique must be a personal choice that is internally motivated. Trust your Biggie when he says that losing 70 lbs during one semester of college will not make you happy.
Now that we have established that your physical choices are yours alone to make and that--while welcome to give her opinion--your girlfriend has no claim to your choice of physique, we can move on to the reality of larger physiques and the practical effects of making them smaller. The most important note is that men of larger physiques can still look good in a gentleman's clothing. The late Sydney Greenstreet was quite rotund, and he still managed to look smashing opposite Humphrey Bogart in films such as Casablanca and The Maltese Falcon. But that isn't even your problem. You aren't an endormoroph like Greenstreet. Rather, your build is quite mesomorphic and you have a large bone structure, so a good analogue to yours would be someone like former NFL Tight End turned analyst Shannon Sharpe. Mr. Sharpe is no small man, but he still manages to be one of the best-dressed athletes in the country. Did you see his Hall of Fame acceptance speech? He's wearing a light blue shirt, navy tie, yellow cornflower sport coat--he's even sporting a boutonniere for good measure! At any rate, Mr. Sharpe is monstrous by normal person standards, and he still managers to dress impeccably. It makes his life more difficult because a large athletic build is more difficult to tailor, but that problem can be solved through a little extra time and effort, from made-to-measure operations to bespoke tailoring. Ergo, the common critique that larger men can't look good is utterly ridiculous.
Your Biggie is not a licensed physician, so I will forgo offering any medical advice--though I will say a few words. While there are health concerns that do play a role in living the life of a Biggie--namely, the ever-looming threat of heart disease--being a 230 lbs. mesomorph is probably not the same thing as being a 340 lbs+ man's man (sunburned after a long day of rugby). Also, in theory larger men should get tired more quickly during physical activity, but I can attest that such is not always the case. Also, any claims about joint pain are suspect: if your body is used to being large, you are unlikely to experience significantly less joint pain upon losing weight because your body has adapted to carrying the weight. This is my experience. (Of course, the inverse is true as well: small men who become Biggies later in life may want to lose some weight because their body has not grown accustomed to life at a larger size, and their joints and fitness may suffer accordingly).
There are some (limited) positive effects of losing the weight--health concerns notwithstanding as noted above. The most obvious of these is that your girlfriend will be happier. Of course, as I made clear above, that is an insufficient reason for you to alter your physique, provided that you have a similar physique compared to when you two began your relationship. (Obviously, if you started out as a twig but then became a Biggie against her wishes, she would have reason to complain). Another practical upside is that clothing will be easier to find and easier to fit. Again though, as I discussed above, larger men--especially athletic ones--can still look outstanding. Any increase in attractiveness to other females is a moot point given your relationship status, and even if females did find you more attractive--a questionable supposition, given your large bone structure and genetic predisposition for size--your Biggie's personal opinion is that physical attractiveness to females is an overrated asset. Properly cultivated game can compensate for lack of Atwatrian beauty much of the time, and with every passing year, it is the size of your stock portfolio--not the size of your midsection--that will matter to the opposite sex.
There are several negative effects to weight loss, as you pointed out. The first and most obvious is a lack of self-esteem. For a bulky athlete, size and strength can impart of air of self-confidence, particularly in the face of other factors that you cannot control (ie: your competition is smarter and wealthier than you). As you said, you believe men respect and fear your size. This is because strength is implicit in size (until you reach a certain level of overweight, of course). As a consequence of losing this weight, men who are smarter than you (or at least think they are) will feel that they are better able to compete with you because you have lost your competitive advantage of being larger and stronger. Formerly, this had been your edge; now, it is gone. If you do work in a male-dominated industry such as investment banking, a large, muscular, former athlete like you is going to command respect solely because of your size (not to mention the synergy you reap from your very aggressive attitude). You are literally generating an aura of alpha. The benefits of this are obvious and do not require further explanation. While the one downside is that people may think you less intelligent because of the dumb jock stereotype, listening to you speak for 30 seconds should straighten them out. And even if they do maintain an air of intellectual superiority, as you mentioned, your physique will continue to instill fear: Wiser men than I have explained why this is optimal: "Upon this a question arises: whether it be better to be loved than feared or feared than loved? It may be answered that one should wish to be both, but, because it is difficult to unite them in one person, is much safer to be feared than loved, when, of the two, either must be dispensed with."
Perhaps the most important point is that it is fun to be large and strong. Doing heavy deadlifts and squats is physically invigorating and extremely satisfying. It increases your muscular strength, which makes day to day tasks such as lifting an ice chest easier. It makes less common tasks, such as moving the contents of an entire apartment, possible. Lifting heavy weights also improves other parts of your life: food tastes better, you will sleep deeper and fall asleep faster, relaxation feels better. All the endorphins you release during a workout leave you feeling great. You will feel more confident and aggressive--important attributes in a competitive sector such as banking. Finally, lifting heavy weights helps increase your testosterone production, which stimulates muscle growth, discourages bodyfat retention, and increases your sex drive. Indeed, lifting big weight makes everything better. (Further questions on this matter should be directed to the hirsute gentlemen who are the proprietors of this website).
But as I said before, your choice of physique is yours and yours alone. No one has a claim to your choice of physique except you. Not your parents, not your girlfriend, not anyone. Only you have the right to choose. (Yes, you should be hearing very loud pro-choice echos). I cannot make this decision for you. I can only provide you my advice and information: large men can look good, and life is better when one is large. The Biggie path is right for your Biggie, and in your Biggie's opinion, right for all men whose genetic makeups allow for it (ie: ectromorphs will small bone structures should not attempt to be something they are not). Hopefully the Biggie path is right for you. Your Biggie certainly believes that it is.
Your humble and obedient servant,