I definitely think that it all comes down to model when you're dealing with the "higher end" bib shorts. I've had Giordana, Assos, Castelli, Louis Garneau, and Voler, and obviously they all fit differently, but even within the same company the fit can be different.
For example, I had a pair of Giordana Silverlines (one of their less expensive models) for about a year. They were great bibs--they fit right, conformed to my body rather than having a predefined shape for me to fit into (something that Assos kind of does), and didn't have much bulk at all. They ended up getting destroyed in a crash so I tried another pair of Giordanas, but this time I went with their Laser series (their second tier model). The fit was TOTALLY different (much too big in the back end, usually a place that I don't have a problem with
) and the fabric was thicker, bulkier...not as comfortable. You just need to watch out for discrepancies within companies like that, as someone said earlier, Assos does the same with their models.
Now, I think that the purpose of the bib is really the big determining factor in fit and feel. My Assos S5 UNO bib doesn't feel like a second skin like my Castelli Body Paint does, nor is it supposed to. It's supposed to have that "all day" feel to it (although I could wear the Body Paint all day too), and the pad follows that function--it is much, MUCH bigger than any other bibs that I have. At first I found it uncomfortable, but I'm starting to adapt to it. Again, it's about the function of the bib, and the fit will *probably* follow.
That said, all the companies, whether it be Assos, Rapha, Castelli, Giordana, or Pearl Izumi or whatever, are going to have different fits. I think it's more about what company fits *you* rather than just simply choosing one.