well it sort of does, I'm not sure you can say they have weaker legs, they certainly tend to have fast legs, though not in kicking as they don't tend to train that.
Fast legs are not the same as strong legs, in the same way that strong legs are not the same as fast legs. Boxers, but the very nature of their sport, are going to have a tendency to train their legs to be fast. Not to be strong. Because fast footwork is an important part of their sport.