It's just human nature to want to see the "bigger/better" things..
Going back hundreds of years with the gladiators, they would pick the big warrior type men and they were championed by the people... Back in boxings early days, it would have been one thing to see men like yourself fighting in the ring against another guy... What a sport... But seeing 2 really big genetically gifted guys going at it would have been a real treat... It's definetely more marketable, and just our nature in general is to want to observe things that are bigger and better than us..
Think of like a pub fight where 2 little skinny dweebs started grabbing at each other and maybe throwing a punch.. It would sort of be classed as entertaining (not that i'm at all a fan of pub fights)... But if the 2 biggest dudes in the ENTIRE pub start grabbing at each other and were about to fight, it would definetely create a different atmosphere in there..
We are still animals by nature, so with a fight between bigger heavier humans, we would perceive there to be more at stake than if 2 little skinny guys are fighting.... TSo bigger people fighting is more interesting and has more value to us...
I guess that's a part of it... I'm sure there are more reasons..
Another might be that 99% of their names are impossible to pronounce.. Heavier guys have easier names I find..

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