Or you can wear road shorts on MTB and look just fine!!

THe Sugoi Neopro short goes up to 2XL, that's a road short, and there's the Devote short, also 2XL, which is a baggy "urban" short.

I think bigger women look fine in road shorts. It's very much too bad that more manufacturers don't offer their entire line in bigger sizes- I'm sure people would be happy to buy them!