bx, yeah ON sizing is alot bigger, I feel that its even slightly bigger than Gap.
BF- BF has never served the purpose of burning calories for me tho I know it does so for some other women. In fact, there are some women who burn calories faster after stopping cause when u BF, your body retains more fats as reserve; Again, this is also only true for some women, and I think I am one of them. For you, could be either due to BF or your own metabolic rate, I guess you'll know know after you stop hee. But hor, looking at your photos, you are def not the big-bone type leh, I think its prob just your genes that you dont gain much weight.