Why is my cat so much bigger than the other one?

Well for one male cats are usually bigger than females anyways. But also each kitten in a litter can have a different father so they might only be half siblings even though they are from the same litter. And plus if one was a runt they can sometimes be smaller than siblings in the same litter and plus other genetics play into their size, not just the parents. So combine with all that and that could be why your male cat is so much bigger than his sister is.

I have 3 cats who are brothers, and they are all different sizes. Panther is 16 pounds, Tux is 12 pounds, and Ghost is only 8 pounds. They had 5 brothers and all of them now are around Tux's size at about 12 pounds, so Panther is just really big and Ghost is really small (he was the runt).

It's genetics - and cats are rather unique in that they are induced ovulaters - meaning that they do not release eggs until they are mated by a tom. In theory each kitten in a litter could have a different father. But remember also it's not just our parents, but also our grandparents that influence what we turn out to be.

A female cat will mate with more than one male if she can. They may only be half siblings. Male cats are generally larger than female cats. If you are 100% certain that they have the same parents, then each of them resemble different ancestors in size.

Its likely they simply have different fathers, as weird as that sounds.

Due to how a female cat releases her eggs when she is in heat all of her kittens can have different dads and are often slightly different ages (Fun fact; Thats why there is usually a runt!)

More info on this if your interested c: http://pets.thenest.com/possible-kittens-one-fathe...