And I'll say here what I said there. . .
While the bullet is moving slower in a short barreled pistol, there are a few other things that make recoil more severe.
1) Less weight and COG further back.
2) Higher pressure differential between the gas in the barrel and atmospheric (creates a jet effect). If you look at a pressure profile graph the longer the barrel the lower the gas pressure at exit. This means less jet effect.
I actually think #2 has more to do with the increased recoil than #1.