I've got a pair of Castelli Velocissimos in XXL that fit great. I'm about 5'11", and 210 lbs or so, with a 34" waist. I find, though, that I have to buy shorts to fit my thighs and not my waist. I'd also venture that the straps are perfect for me.

I wonder how much sizing differences are because of differences in build, especially with stretchy apparel like cycling gear.