Several years ago, I looked up how big the old APS film cameras were, under the assumption that, given that a dSLR is the same size as an old film SLR, it will eventually be possible to make an APS-C digital camera the same size as an old APS film cmaera. This turns out to be awfully small, especially with a prime lens. I also eventually read about the Olympus PEN series. And the Pentax Auto 110 SLR system.
One of the central arguments of the 4/3rds mount was that digital camera sensors were destined to stay small, thus, a new mount for these smaller sensors, with a smaller lens throat and registration distance so that the lenses could be smaller and lighter, only covering that 4/3rds mount size. Of course, Olympus kinda screwed it up.