I haven't measured the difference, but I'd say the XDm is about 1/4" longer to the bottom of the mag. With the XD sub compact I need a pinky extension the keep my pinky finger on. With my XDm I can get away with just the regular flat plate on the mag and my pinky is just barely on. That's how I carry it.
I was going to go with the xdm compact, but opted for the full size. I couldnt get my pinky on the compact grip. I know they make extensions, but whatever ya know.
Im sure you could call 1-800-680-6866 SA Store and get the dimensions
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