I have a couple of "redundant" lenses - 50mm f1.4 and 28mm f3.5, across various mounts that I can compare and it sure looks like there are quite obvious differences in size. Probably an aesthetics consideration in the design to match body and lens. Obviously the Pentax M lenses for the M Series were also designed small just like the OM system.
There are special off brand lenses (also available in other mounts) that are worthy of consideration for use on the OM - the Kiron 105mm macro and the Vivitar Series 1 70-210mm. I am not sure how much they could have shrunk the design though but they're exceptional lenses.
BTW, the OM2, 3 & 4 have the most sophisticated metering system as they continuously monitor exposure while most all others meter just before shutter fires. For instance on long exposures, if the light gets brighter on the target after the shutter has fired, it will close the shutter sooner accordingly. Most all others - even latest and greatest, will simply stay the course and obviously overexpose. Of the classics I own and others I've tested, only the Pentax LX works this way too.