Chawin and Lesnar are the same size. Difference is Carwin actually has skill. I hope Lesnar gets KOed so bad he quits the sport. He's really giving MMA a bad image...
Lesnar is bigger, no doubt. The dude cuts 20 or so lbs to make 265 while Carwin weighs around 265 walking around, you will see on July 3rd. Carwin is barely larger than Couture and his "6'4" stat is heavily exaggerated, he's closer to 6'1-6'2.
Plus Lesnar was a Division 1 Champion in wrestling, Carwin was a Division 2 Champion.
His BJJ world champion coach Rodrigo Comprido (google him if you don't know his credentials) states that he is one of the fastest learners and progressing well with the submission and positioning aspect of MMA.
The only advantage I see Carwin having is the timing he has with his hands to land big shots which he's been doing lots lately.
Carwin's only notable opponent besides Mir was Gonzaga, Lesnar has fought Herring, Couture and Mir aswell, so in that respect I think they are similar but Lesnar has the slight edge here.
I can see Lesnar grapple fucking Carwin, I doubt he'd want to stand with Carwin just for the risk factor. Lesnar should be able to handle him in the wrestling, gnp and jiu-jitsu department so thats where I see him taking the fight. I predict the fans will boo a lot in this fight too.
Anything can happen though, It's MMA.