I don't know, I've never been 300 lbs before. 265 is my biggest. I've noticed that if I'm getting winded easily, I just need to throw in cardio a few times a week. If my joints ache, well, that's a price you pay for doing anything that takes a toll on your body. When I was playing basketball in college, my joints used to ache just as badly after several hours of practice. Ok, sure, you fart and shit a lot. If a few extra bathroom trips each day is enough to steer you away from your goals then I think you need to look at your work ethic. I like to eat a lot. I come from an italian family where large quantities of food have always been the thing so I never really view it as "stuffing myself", plus when you are on certain things, you are much more hungry and more often so therefore you want to eat more.
I'd be curious to see how many times I've made the following statement on this board, but here goes again:
Set your goals and act accordingly. If you want to be a pro bodybuilder, you gotta accept what that takes. If you just want to be a top notch gym rat, but don't want any of the little annoyances that come with it, then you need to find a balance and act accordingly to that.
Lastly, when you ask why someone would want to be so big? It could be for many reasons ranging from competitive to mental health. Why would people want to base jump or sky dive? Why would people want to enlist in the military full well knowing that they could die at war? Why does anyone do anything? It's a matter of choice and doing what makes you happy. Some people may not agree or feel as you do, but it doesn't mean that what you are doing is wrong.
Food for thought kids.