Not according to Mackie Shilstone: http://www.si.com/vault/1985/10/07/620972/a-champ-with-strange-ideas
He wouldn't have just "put on 25 pounds of muscle", he just wouldn't have cut weight + also bulked up. He'd have to put the weight on to fight Holmes in order to meet the HW limit + also ensure he had the strength in the clinches and also extra power to bang with the bigger man.
Everyone's circumstances are different. If Larry hadn't have been as big, Mackie may not have had Spinks at a solid 200-205 pounds. Additional mass can affect your stamina, especially if it's your biggest asset like Rocky. Rocky bulking up his small frame may have affected his performances. Ricky Hatton had an optimum weight of 140 pounds. He was never the same force higher than that.
Precisely. No reason to believe that Spinks would have weighed anywhere near 175 if he had been fighting even 190-200lb men his whole career. He stayed small so that he could fight smaller men.
A full-grown professional athlete put on 20 lbs of muscle or so in 3 months. Nothing out of the ordinary there.
I hear you, but I would have to say that Spinks was tall & lanky. He also never looked "bulked" as much as just more filled out. But honestly, anything is possible. I assume most fighters now are "taking vitamins".
I don't think he ever "drained" to get down to 175. Sure, he cut weight, but that's meaningless. Duran 'cut weight' to make 154 pounds. "walk around" weight means almost nothing, some guys are just more disciplined than others. Spinks was very comfortable at 175 and under, hence why he weighed in at 170 1/2 when he faced David Sears, just 7 months prior to the Holmes fight. Obviously Spinks put on the 25 pounds because being heavier is an advantage. It's not even primarily to do with extra strength, power or punch resistance, it's the simple fact of being a substantial weight. For example, take two fighters. One is 175 pounds and the other is 225 pounds. It could possibly be that the 175 man is stronger and more powerful than the 225 pound man, but even in such a case that advantage is offset by the fact that he's up against a heavier object. The heavier object wears away his strength advantage pretty quickly. And usually the bigger man is the stronger man anyway.
And Spinks bulking may have affected his as well. When you watch Spinks at HW and watch him at LHW, is he as good a talent? Evidently not. But he had to add the weight to compete in a heavier division. Rocky wouldn't be 189 now and be a HW, it just would not happen. He either adds the weight or he never attempts the division.
As for Marciano, he could probably easily hold his weight at around 200 or 205 and still be 'in shape', but he didn't have the height, reach, style and speed to succeed against the best men of the 215-225 pound range.