I think its that he doesn't use his strength correctly, "stamina" really isn't an issue, he's so big he should just be quick on this shit.
That's also true - He uses his strength too much. He's use to Street fighting, throwing his punches hard as shit and connecting. Well in the UFC, it's not like that. You throw a punch that's not suppose to be there, then you get your ass lit up. It's simple as that. He can't keep up when he's missing his punches with all that strength he has. You'd think he would kick ass, but when it comes to other fighters that have been training for things like this, he just gets blown away.
He's a good Street Fighter, and should continue to do that. Wonder if he will after getting cut from the UFC. That has to suck :/