Why did sub-190lb heavyweights disappear after Marciano's era?

Michael was 199 3/4 for his first match against Holmes.

 

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Better nutrition my arse, you're telling me Tyson Fury eats better than Marciano did......

 

When 190 heavyweights were on top, boxers were still meeting a healthy height to weight ratio. In fact all the way to 210lb most heavyweights were still making healthy height to weight ratio usining current guidelines.

I think what happened was that when 176lb Micheal Spinks was successfully manufactured into a 200lb heavyweight, without losing speed, regular heavyweights began to manufacture into super heavyweights.

It's not necessarily cheating. Or PEDs. Just taking advantage of more tailored training and diets to increase size.

Perhaps when this happened coaches were recruiting taller and leaner guys to develop more size into the Amateurs?

All these sizes of guys had always been around In boxing. Sports training changed.