Two factors. The lower slide mass is balanced with heavier spring weights stuffed into a smaller available area-not a good formula for longterm spring happiness. The other problem is the steeper angle the barrel must pivot to in order to travel the same vertical distance for unlocking. Picture the bushing as the pivot point and it is easy to see the breech will have to move through a greater angular arc to open the shorter barrel. Other designs get away with this better because their locking surfaces are much shorter than the 1911's three lug system.