It makes me laugh that you're comparing Nelson to Lesnar. Nelson should start with losing about 50 lbs of fat, then cut 10 to make 205 and fight around 210-215. He's a talented fighter (with great BJJ), but he's somewhat outclassed by HEAVYweights. A clean diet would help him a ton. He clearly has the cardio and drive to be dedicated, he just needs to apply it to dropping weight. It might take a few more losss for him to realize his lean body mass isn't the same as the rest of the big boys in the HW division.
And I assume the Mir/Nelson you're talking about is this one? like... 2003? Really? A grappling tourney? And I'm pretty sure Lesnar put Mir on his back before Nelson did.
http://www.youtube.com/watch?v=VPTgegc19lY
And BJJ brown belt means NOTHING about take down defense. BJJ is a fighting style from the ground - top or bottom. Rarely are BJJ skill exhibited on the feet.