bart.lacks1322229wrote a review of Castelli Presto Short - Men's on August 3, 2011
Interesting...I see 2 guys comment that they have a 36" waist. One says kinda snug. Other says maybe he could have gone to a smaller size.
So, it must have to do with the shape of the body. I also have a 36" waist, and so I ordered the XL.
The shorts fit fine in the waist, and I really liked how the legs felt and looked.
But the area through the hips was too small on me -- I guess I'd have to go with an XXL, but I just ended up returning the ones I ordered.