Question about cats weight

[GEM3]

We have 2 adorable siberian cats that we got from 2 different Siberian Cat breeders. Both kittens are super-pretty, healthy and with good personalities. The kittens now are 7-7.5 months old. They are so different and, if we wouldn't known better, we would question if they are both the same breed. One is beautiful silver mac-tabby w/ white and another one is gorgeous seal-point. Both are boys.
So here is my question -
the seal-point is a much larger cat. He always was much larger. He is a really big-boned. He now weighs almost 13 lbs! The other 7 months old boy is normal - weighs about 8.5 lbs.
Is it OK for a large cat to weigh almost 13 lbs at 7.5 months old?
Can the kitten be obese at such age?
We give them both dry and can food twice a day and the interestingly the smaller kitten usually eats more than the larger one.

Thanks so much in advance for your comments!

[Arianwen]

I have four young cats - to say they vary in size and weight is an understatement. None of them are fat but Baz is tall and heavy boned - his full sister is a dainty little thing. Without knowing the cat, it is hard to say.

[Marcia]

Cat breeds are like human nationalities. Not all Indians look alike; not all Chinese look alike, etc. Some are smaller, some are taller, some are fatter, some are ..... well, you get the idea. I guess you could look some Cat Fancy list of "ideal" physical show qualities of the Siberian breed and see if either one of your cats compare.

[GEM3]

Thank you all for your posts!