HW's are no longer the lumbering people of old where they have bad technique but can knocked you out in one punch if connected. Now the new breed of HW's like Todd Duffee, Cain, Lesnar, JDS, Bigfoot, Carwin etc where guys not only were getting bigger, but stronger, and faster. And now they also have technique to combine with it. They have crisper striking and now also have a ground game. Back in the day a HW couldnt do anything with a ground game and suspect cardio. On the other side of the spectrum, LW's couldnt knock out anybody, but they have great technique and cardio and used that to win.
The lower you go technique becomes more of a factor, the higher you go power becomes more of a factor. Fedor just cant keep up with HW's that are not only getting better physically but having the technique of lower classes. It was like fighting a slow Hippo before, but now its like fighting an Elephant with the speed of a Giraffe.