You don't get fatter when you sit down. It's just that the fat on your stomach is getting curled up ontop of it's self and makes it look like you are fatter. It also has to do with a little thing called Gravity, it displaces you fat and makes it look like you have more of it. Same concept as when you lay on you back you look skinnier!
I've noticed that when I lay on my back I look skinnier