Does anyone know the dimensional differences of a Glock 19 or 23/32 compared to a USPc in 9 or 40?
I am most concerned about the width and height.
With the flat floorplate on the USPc magazine, height is about the same. I have the .45, and the only difference between it and the 9mm/.40 S&W gun is the barrel/slide is a bit longer, and the grip frame is a bit deeper to accomodate the longer .45 cartridge.
The USPc is a bit "fatter", width-wise, than the Glock; through the slide area only. Glocks (except the .45 GAP models) have the same width from top to bottom; the USPc's slide is a bit wider than the frame, a'la Sig P229.
The USPc is perceptibly "bigger" than the G19/23; so if size is your most important criteria, go with the Glock in a 9mm or .40 S&W.
The .45 USP Compact is where the big differences are. The Kahr P45 and a couple of whacked 1911-style pistols might be smaller than the USPc .45, but that pistol is the softest-shooting .45 I've ever fired, in any size handgun. A pal has the Kahr, and I've shot it quite a bit and tried to warm up to it, but just can't. I find that little pistol distinctly uncomfortable to shoot.
Anyway...if smallest size is your main concern, the USP Compact is NOT the smallest out there. It IS the "best", IMO; small enough to easily conceal, and leagues above the others in terms of fit, finish, and overall quality.