Does my rat have a growth defect?

I bought two rats about 1 month ago. They are both brothers and were born in the same liter (they're names are Toulouse & Ziegler). When I bought them they were about 7 weeks old. Ziegler was much smaller than Toulouse, almost as if he was a runt. However, they are both starting to grow and Ziegler has grown to be MUCH bigger than Toulouse. The weird part is, Toulouse has barely grown! Does he have some kind of growth defect? He's only grown a little and I find it odd that Ziegler was much smaller in the beginning and they are both the same age. Anyone else experience this with their pet rats? Thank you