sorry all.
the correct answer is......GENETICS :p (lean people stay lean, fat
people stay fat, moving up or down within their "weight
bands")
and.......lifestyle!(to a smaller extend, for example frequent
snacking on junk food or late-night suppers)
eating heavily then sleeping is a g8 wway to gain fat 
i believe that the calorie intake/calorie output balance only
detemines your overall fat/muscle proportion, it won't really
affect your weight much.
am I right? if i'm not, please flame me :p