I got these as a gift for my BF and I was concerned about the sizing, but they do stretch quite a bit without being too constricting.  According to the Under Armour size charts, he should wear a XXL based on his waist circumference (44 in.), but if you go by his US pants size (36-38, worn below the belly), he should wear a L.  I decided to go with an XL and they fit just fine.

He wore them when we went hiking yesterday and he was far less chafed than usual and had zero jock-itch.  The only thing keep this from being a 5-star review is that he wasn't as dry as he'd hoped.  But then again, we were hiking in a sub-tropical jungle and he is a sweaty guy.