I bought a pair of Ultras last year, and based on their sizing chart I'd be a size M, considering that I have a 31" waist. I found them extremely tight and actually kind of hard to get into. Some time last fall I got my first set of bibs from my team, size L, and realized how much BETTER they fit. I compared them to the Ultras and there was a massive difference that had absolutely nothing to do with one having straps over your shoulders and the other not.
Thanks to Performance's return policy I was able to return the Ultra M's and get L's instead. They fit substantially better, but according to Performance's website, this means I must have a 36-38" waist. That ain't right . . .