I've never heard of someone gaining weight so quickly (outside of athletes cutting weight that is).
So when you personally lose or gain weight, it's all just water?
That's one way of looking it, although it's not that simple.
If you eat too much and create new fat cells, you have them for life. They're really hard to shrink, and they expand very quickly (if shrunk). That's why almost no obese person permanently sheds the weight, yo-yo weight is very common in people working hard to drop the weight, and almost every fad diet guarantees rapid weight loss in the first weeks.
Once you're fat, you're fucked. Yes, eating properly and getting a lot of exercise are incredibly important, but you'll only partially manage the weight. They're important to be healthy.
I know a lot of people, especially on Sherdog, hate the "love what you look like" message, but I have mixed feelings about it. I dislike that the message is too simple and never mentions that if you aren't fat yet, be smart and don't get fat. However, everyone mocks fat people, and living fat sucks. A lot of people are miserable and are ALWAYS going to be fat, so I get that some people out there are sending those people a somewhat positive message about them. I just think that message should include how important it is to eat properly and exercise. Accept how you look, but don't make it worse, and for God's sake get exercise. I have almost never seen that message.
And the people that celebrate obesity? They should be stopped. Fat isn't beautiful. It isn't healthy. There is a minuscule percentage of the population that actually wants to be obese, it's a fringe fetish. Just about everyone who is fat would prefer to be otherwise.