I had both Pentax MX and Olympus OM-1/OM-2 cameras. The MX is very slightly smaller; the Olympus felt better in my hands. Both were excellent cameras.
But honestly, in this camera class, I prefer the Nikon FM2n. And my current small mechanical SLR, the Leica R6.2, is not much larger than the FX if at all, and has the best lenses.