Now, part of the reason that the page count went up on both the 3rd and 4th editions is because art was added, both times. Champions is a super hero game, and super heroes are a very visual genre. Adding art made sense.
When DoJ took over, the amount of art dropped. They were trying hard to push the Hero System as a generic system, and, to my eyes it looked like they were trying to hide the system's super heroic origins.
As I said, Steve Long had been a lawyer. It is evident in his prose style, and length. It also meant that he became, literally, the ultimate Champions Rules Lawyer.
Now, for 5 editions, the Stat block for Champions/Hero hadn't changed. Whatever rules changed between editions, you could still use a 3rd edition character while playing 4th edition. You could still use a 4th edition character while playing 5th. Hell, you could still use a 3rd while using 5th. My player group did, from time to time.
Not so with the 6th edition. Long & Co. decided to change the stat block. They changed what had been the "figured characteristics" (the characteristics whose base values were base on main characteristics) into just characteristics. Ostensibly, it was to "make things fair"; there were some mathematics quirks that made it so, for example, being strong and tough was more cost beneficial than being weak and tough.
They renamed several powers for the sake of renaming them, or, again, to conceal the system's super-heroic origins.
That's all I really know for certain. Honestly, between the lack of backwards compatibility, and making the main rule book two ridiculously sized tomes, that was enough for most of my group. Most of us were unhappy with just how ridiculously rule heavy and "crunchy" Hero had become. We had used Champions for nearly 20 years in spite of the crunchiness of the system, not because of it.
We decided to stick with 5th edition. No, we actually slipped into when we jokingly called "4.8" Mostly using 5th, but in a few cases going back to 4th.
W: Professor Plum, you were once a professor of psychiatry specializing in helping paranoid and homicidal lunatics suffering from delusions of grandeur.
PP: Yes, but now I work for the United Nations.
W: So your work has not changed.