I have a fat and a skinny also.
Elliot had HUGE paws when he was a kitten so we knew he was going to be a big boy, but aerowyn on the other hand was the runt, and at five months and even before that her paws were basically perfectly perportioned (which isnt a big tell of how big the cat will get but it is a clue) she will be one in a couple of days, and she only weighs five pounds. I dont lik eher being so skinny but I cant seem to make her fatter! I let them graze all day but if I were to switch to a higher fat food, elliot would get fat :O I was feeding her kitten food up until six months but then she started eating elliots adult food and he ate her kitten food down and wouldnt touch his own, and as he was getting fatter on that I opted to just stay with adult and give aerowyn wet food when elliot wasnt watching. (The easiest thing is to get him out onto the screened porch and then feed aerowyn) But she still stays skinny. Its just the type of cat syou have... just like you have people like shaquile o'neil (probably spelt that wrong) you have tiny little midget people!
YOu wouldnt expect a midget to weigh 230 lbs or something would you? :P Or Shaq to weigh 45? I myself like a bit chubbier cat though, I dont like it when you can feel their backbones or hip bones.