The kittens may have different fathers and this may also be the reason that their body types are different. My Pearl is much longer and thinner than Starr. She's very energetic and doesn't eat very much either. Starr has gained a lot of weight due to the pred he's on so his real body type is much slimmer. Ziggy has become a very large girl so some people may think that she's a male. I wouldn't worry too much about the kittens. Just like people animals come in all body types.