They both appear to me to have naturally very skinny legs but Joshua has obviously worked hard on those quads, still has tiny calves though. Compared to someone like Joseph Parker or basically any Polynesian ever they both look disproportionate and top-heavy though.
Wilder's legs are skinny. But boxers shouldn't have really muscular legs. They will end up slowing you down. All the best boxers have skinny/normal size legs. :yep
I'm not sure if you can work your calves though (to get bigger that is), mostly genetics from what I heard.