Weight Difference Between Littermates

We adopted 2 male littermates, Leo and Clarkson, about 6 weeks ago. The shelter had them listed as 13 weeks at that time. When we took them to the vet the next day, Leo weighed in at 3.8 pounds and Clarkson was 4.5. I thought that almost a pound difference was quite a lot at such a young age. They haven't been back to the vet yet, and I have no way to accurately weigh them at home, but since then, the size difference in them has gotten only bigger. Clarkson is just a big, lanky looking fellow, and Leo still looks very "kitten." Leo has grown some though, because I've had to loosen his collar up a notch or two. 

I know adult cats come in a variety of shapes and sizes, but how early does that start? Especially with brothers, I would have expected them to be a lot closer in size for awhile, but I'm not experienced with cats so I could be way off base there.

You can often see a difference in kitten size at birth, and as they grow, genetics tell more and more in the cat's size, shape, etc.
 

Just weighed the boys tonight and just to update...they're something like 9 months old now and Leo weighs in at 6 pounds and Clarkson is 14 pounds. Holy geesh lol. Clarkson is by far the biggest of our 5 cats, and he's not overweight at all, just...well...HUGE!

Awww, they sound adorable! And my sibling kitties were very similar with their size differences compared to yours. Caspurr and Rolly were brother and sister (Caspurr is no longer with me) - Caspurr was always tiny, about 6 pounds and very long and slender. Ate like a garbage disposal and didn't gain an ounce. Her brother Rolly, on the other hand, always seemed to have a slightly below-average appetite and he held steady at 17 pounds for years. Caspurr also never had hyperthyroidism, so I guess she just had a naturally high metabolism. Amazing how cats from the same litter can have such different weights. It's been almost 15 years, but I remember these two when they were babies and Rolly was a little bit thicker than Caspurr - he was named after the fat puppy in "101 Dalmatians" and kind of grew into his name, which just makes his name that much funnier. I don't mean to thread-jack about my kitties - just saying I know what you mean about the vast differences in body sizes between sibling kitties.

I had two brothers who were very different sizes when they were dumped on my doorstep at 5 months. They never lost the size difference, until one died at 2 years old (of poison, nothing to do with size). The other is still with me at 7 years old, and is also huge, muscular and walks like a lion. He weighs in at 8 kg, none of it fat. His brother was lean and fit, but much smaller, almost half the size.