Frank Mir's cardio, for whatever reason, isn't the best. Maybe he hypes himself up too mcuh before the fight. I think Cormier is going to wear Mir otu standing, or KO him, before he a tired Mir has any chance to do anything on the ground. Just follow the JDS routine. Sprawl and Brawl lol.
That's because his body isn't meant to be 260lbs+. His muscles (the big oxygene consumers) grew, but his lungs didn't.