Hi sylar
It is my just personal perspective not based on any scientific study or evidence.
I think it is because people lengthen femurs (generally 1 mm/day) faster than tibias (0.75 mm/day). Sometimes, the difference in the lengthening rate is even bigger than this. Plus, people lengthen more amount in femurs (7-8cm) than in tibias (4-6cm). This basically generates much bigger gap between the bones in faster time period. Bone consolidation is in general coming much later. Therefore, bone cells try to find their ways to make a bridge between the bones. Bone cells in femurs seem to travel wider range than ones in tibias. As a result, they form bigger or more likely wider bones than their original size. However, bone cells in tibias seem to travel less wider area and the gap between bones is also small. As a result, new bones seem to stay where they are supposed to form. Anyway, it is all my theory.
The thing is this doesn't really matter for any thing. The bigger bones don't mean stronger than they used to. The bigger bone will not show up outside anyway. So it is not a big deal.