Is it hard/soft/pushable?
IDK a medical reason but I could see if both were the same, maybe it could be about a weight issue. Otherwise, that doesn't really make sense. Does it hurt?
When I was down from the 300s to the 170s, I had really saggy upper legs/knees but nothing like that. I'm not sure I believe your doctor. So many of us are blown off by our docs for being heavy, it makes me sad.