The fighters of the past may have had better stamina then the fighters today because they simple trained and fought more often AND more consistently.
I was watching a Sport Century show about Rocky Marciano where his friends, family as well as coaches confirm the fact that Marciano was a fitness nut and essentially trained ALL the time even when he didn't have a fight. Rocky apparently care about the shape he is in. This was also stated in a biography about Rocky Marciano.
One of the links above is to an old strength and conditioning magazine where Jack Dempsey talked about his training. He, like Marciano, liked to keep in shape and essentially trained ALL the time even when he was competing.
The problem with todays boxers, if they actually have a problem, is they only train when they have a fight. They only work when for the two or so months of training camp. this would account for the flactuation in weight.
I serious doubt that Marciano would go into training camp with losing weight in mind. Why? Because since he was already fit and at his fighting weight before training camp there would be no need to lose weigh during the training camp. The training camp wasn't meant to get into shape BUT rather to take one shape to another level, that is, to peak it.
Also if Joe Louis actually said that I think it is a stupid statement which has no logic to it. George Foreman is simply a larger man. I think 220 is a good weight FOR HIM and a natural weight for him. For Foreman to come in any lower than that is just plain dumb. What would be the point? See, I think Joe Louis is trying to impose onto Foreman a body weight which may have been good for Louis or perhaps some other heavy weight boxer but not necessarily for George.
If George was fit and trim at 220 then that's the weight best suited for him, he should have to come in any lower than that.