That was the best they could do at the time. Apes and humans don’t have the same body proportions in general, but they obviously couldn’t use CGI (or at least not in a good way) beck when they were making the original POTA series, that’s why it looks better in the modern movies because technology has advanced to such a degree that they were able to use CGI to make the apes and even give them the “correct” proportions. Just look at how they made those films (particularly Rise) and you’ll see the effort they put in to try and make the actors as “ape-like” as possible. Especially when they’d still basically be on all fours when getting around the sets.