The Rohrbaugh and the new PF9 if and when it finally arrives are both single column magazines. The P11 has to be the thinnest at 1" and carries up to 12+1 rounds in a pocket size package. You can get S&W 5900 series magazines with an adaptor that extends the grip for it, too, making it a 15+1 round gun. I tried aftermarket Smith mags, though, and they didn't work. I've been assured that S&W mags do work in it, though. That is one of the selling points to cops looking for a BUG for their Smiths. You could carry the Smith mags IWB easier than carrying the Smith and have full size firepower in a very thin, small handgun.
I have a P-11 and use S&W 469 series 12 rnd mag and they work exceptionally well.