Bodyfat percentages are not everything. Fedor has always looked chubby. It's just his bodytype and genetics. Like Igor Vovchanchin. Josh Barnett always looked fat, but was in good shape for most of his career.
Fedor fought at the weight he has been consistently fighting at, and the weight he feels most comfortable. Sure he could probably cut down to LHW, but he stays at HW because that is where he feels comfortable.
Chael fought at a weight he does not normally fight at. I watched all the behind the scenes stuff. Chael tried hard to gain mass the right way. He said eating a lot was actually a struggle when he was also trying to keep good cardio at the same time.
This affected GSP too. GSP said he just didn't feel right at 185. He said he couldn't move as well, and just felt clumsy compared to fighting at 170. And GSP has far superior genetics to Chael.
Chael is an average sized MW...hell I'd argue below average by today's standard of monster 215 to 220 plus guys cutting to 185.
Just like Conor McGregor has no business at 170. I still give Conor credit for stepping up and fighting Nate Diaz...a guy with the body type more suited for 170 than Conor who is only 5 ft 9.
I think Chael and Conor are both in similar situations in that the weight cut for their appropriate weight class drains them a lot, whereas some guys seem totally unaffected by huge weight cuts (like Khabib, who is actually bigger than GSP, unless GSP is lying to us).