I can't do you a side by side, since I traded off my 3913 years ago, but I can give you my perceptions. The 3913 is more compact, being a single stack 8rd. mag. The grip is thinner and shorter, and the frame/slide/barrel are shorter as well. The 3913 is a nice flat pistol, that conceals easily, but is still large enough to handle well. The M&P9, which I was recently issued, is larger in all dimensions. That said, it still works well for me so far as a concealed off duty piece as well as duty gun. It is slightly smaller than the Beretta 92FS I carried prior to the M&P, and while I again have no side by side, I would say it is only slightly larger than the Glock 19. It is light, and with the small grip mod. and a hogue rubber grip sleeve, I think it feels great. Hope that helps!