Wider seat, wider stance, pushing those legs out and putting stress on your hips and knees.... I'd be surprised if you knees don't start hurting too (or already are?). I feel it instantly when I get on a Kitty.
My observation anyhow.
I think your on to something. You should have seen the wear spot I had on my ankle due to the difference in sleds. What thaaaa???? Don't say knee!!!!!!!!!!!!!!!!! Yikes!!!!!!!!!!!!!!!!!!!!! Not yet thank god. Just everything else. lol
BTW, my sled has a BOSS seat on it and it is shaped kind of funky.
I'm going to ride the hubs sled today. He has a WRP on his. Little higher. That my be my solution. Then you'll hear me complaining about tripping on my seat when I hop side to side. lol