I've never had any problems with my mtb baggies getting caught on the saddle of my mtb. The only problems I have had have been with liners that got all old and stretched and stuff (which will happen to pretty much anything evetually). Maybe it would be more of a problem with the less upright position on a road bike? I don't know, and I guess I don't know what kind of bike you are riding either. Anyway, I'm pretty sure they have some plus-size mtb shorts right here on TE, if you think you would like to go that way.
Yep, here they are: http://www.teamestrogen.com/products.asp?pID=14891
Drink coffee and do stupid things faster with more energy.