I should mention that on many pistols, it’s much easier to insert a fully-loaded magazine when the action is open than when it isn’t, and I’ve handled pistols where inserting a fully-loaded magazine far enough to engage the catch that holds it inserted is noticeably difficult when the action is closed, but easy when it’s open. I would estimate that on a 9mm, one could shave about 4.5mm off the length of e.g. a 7-round magazine if one were willing to have the gun be a 7-round total capacity rather than 7+1 (or, conversely, adding about 4.5mm to the magazine would greatly increase the convenience of loading the pistol with 8 rounds).
Go find a pistol designed such that it will not accept a magazine with the action closed, identify it, then report back. Until then, your theory does not hold water.