Dimensions of some S/A semi-automatics
6.25" oal 4.75"H 26oz. ----- XD 40SW Sub-Compact (3" barrel)
6.75" oal 4.75"H 28oz. --- XDM 40SW 3.8 Compact
6.75" oal 5.75"H 29oz. --- XDM 40SW 3.8
The XD Sub-Compact is 1/2 inch shorter than the XDM Compact, It would be expensive to try to shorten the overall length of an XDM 3.8 down to Sub-Compact length.
If you just need the 1" less height at the grip, you would be better off, just trading your XDM 3.8 for a XDM Compact.