The Boberg XR9-S Shorty was designed to duplicate the ballistics of the G26 "Baby Glock" in a much smaller form factor.
At left you can see the size difference between the Boberg XR9-S Shorty and the G26.
The XR9-S Shorty is a fraction of the size of the Glock 26.
The XR9-S Shorty delivers the same amount of stopping power as the G26 because it has a similar-length internal barrel and is able to shoot high-pressure 9mm+P ammunition, just as the Glock can.
The Boberg XR9-S Shorty delivers G26 power in a gun the same size as the smallest pocket pistols such as the Rohrbuagh R9/R9s and the Kel-Tec P3AT.
This overlay shows the Boberg XR9-S compared in size to the Rohrbaugh R9/R9s. The Rohrbaugh has a reputation of being the "smallest of the small" in 9mm.
The Boberg XR9-S is a different class of pistol, not because of the Boberg's overall smaller size, but because of its much longer internal barrel and ability to safely and routinely fire 9mm+P ammunition (which Rohrbaugh does not advise for its pistols).
These engineering differences result in a dramatic increase in muzzle energy of the Boberg XR9-S over Rohrbaugh 9mm pistols.
This overlay shows the Boberg XR9-S compared in size to the Kel-Tec P3AT.
The XR9-S delivers more than double the muzzle energy (up to 499 ft-lbs of energy) than that of the P3AT.
Because of vast difference in stopping power, these two pistols should really only be compared in size.
While the XR9-S is slightly smaller in length than the P3AT, it will be heavier and slightly thicker due to the larger and more powerful 9mm+P ammunition it fires.