It is not a shorter barrel, actually. The HK P7 has a full length 4.2" fixed, polygonal barrel, but in an extremely compact pistol.
That means you have full sized pistol accuracy, power and range...but in a subcompact size. So while a PM9 is making due with a tiny barrel that is going to cost you 150-200fps in velocity, the P7 user has a full length polygonal barrel that will let his ammo perform to it's max. Where your J frame is making due with a 2" barrel and is producing under 300fpe of energy even with hot loads, the P7 with hot +P+ ammo can be prodicing over 500fpe of muzzle energy- which, by the way, is solidly into .357 magnum territory. And the P7 carries 9 rounds vs the J frames 5.
Yes, the P7 is heavy, as it is all steel (to me, that's a plus- not a minus), but as most people who CCW tell you, a properly designed holster renders that point moot.