atatomy question?

I am studying cat and dog anatomy for my program right now

I have 3 kittens 2 have a normal build the third does not feel like the same animal if you pet her shes solid like a brick? they are from the same litter, the same age but one has the body of a mini big cat.

Anyone know why just curirous

It's possible that more than one male cat mated with the female, resulting in a litter of kittens fathered by 2 different male cats.

Or it could simply be genetic traits dictated by the female and male cat's genetic history.