Awww little Carmela is all packed and ready to go!

I have a question, and it's okay if I don't get an answer. But, I see the vet put their birthday at 4/14/10 and they weigh over two pounds. My Jax's birthday was estimated to be 4/5/10 and he weighed in at 1.6 pounds at the vet on Tuesday. Is Jax just small for his age? Or are these babies big? I know they look at teeth to determine age, but a whole pound difference when they are 10 days apart (approx.) in age.