getting fat - stupid biology

i'm fucking getting fat. fatter and fatter.
and it's really wierd! i used to eat like two pizzas everyday, and i wouldn't budge a gram in weight.
now i've started to eat healthier (like sallad for lunch), doing yoga, and working out a little. i do 60 sit-ups every morning and night - and the result is getting fat! it's all the stomach too.
my hypothesis is: since i've started to eat healthier and cutting down on meat i don't get the same amount of fat and overconsumption of nutritions as i used to get, so the body feels that it has to build up a reserve to help me survive...

stupid body!