Stamina issue's in boxing?

Some guys are too tense or on edge, you can see DLH like that in the early rounds and then by the end of the fight he is slowing down. But as a stamina issue its exaggerated a little bit, its not like he was collapsing in the corner, just that he had slowed down his pace.

 

Being relaxed with proper pacing is what they can do once they're inside of the ring to make sure their stamina stays cool.

The bigger muscles are, the more oxygen they need to function. Being buff and bulky wouldn't just hurt speed, smoothness, and slickness but also stamina.

Genetics play a role. Zab Judah is THE fast twitch muscle fiber guy. He is extremely quick with amazing handspeeed and reflexes but can't ever go the distance.