Why its so common for heavyweights to be out of shape in fights?

Go back to tape from the 70s, 80s and 90s and early 00s.

You had guys like Mercer and Witherspoon, fixtures in the top 10 and they were never in great shape, yet gave some of the top fighters in those era thier toughest fights, and in some cases, embarrassing Ls.

Rahman was never a beauty either, yet he was right in the top 10 after his stoppage over Sanders.

Lewis is said to be fat in 04, but he looked almost the same after his win over McCall, and was still in great shape compared to the likes of Johnson.

 

It has already been said, there is no weight limit. Yes you want to be in top condition but if you are good at 240 then why get to 220? It's probably a mind game too because you could lose strength in the clinch and what not. Ray Mercer was in good shape in his younger days. Not sure about Witherspoon though.