Unless you are genetically predisposed to large muscle mass, you will not have large muscles and be doing the amount of training they are doing.
Many fighters are very lean, which makes their muscles look larger, but realistically their fat free mass to body weight ratio is probably not too abnormal.
However, as you get up into the higher weight classes (170+) you start to see what is clearly abnormal fat free mass to body weight ratios. There’s a reason why boxing has LHW at 178, because those classes were created at a time when there weren’t a bunch of ergogenic aids to prevent muscle wasting at the high workloads they are performing.
This is Jack Dempsey, one of the greatest HW boxers that ever lived:
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That’s what your generic, athletic guy will look like doing that much cardio. Granted, he probably didn’t lift weights, so his muscles might be a bit more round and hypertrophied if he did.
But compare this with GSP:
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I think the difference is readily apparent.