An APS-C lens is just going to be bigger than an M43 lens. That means you can carry more lenses for an M43 camera. I have an Olympus/OM camera plus three lenses and charger, and it all fits in my flight carry-on luggage together with a laptop, underwear, and an extra set of clothes. APS-C and full-frame are cool, but they're annoying to carry around. For travel, nothing beats M43.

They are a bit smaller but, to me, not that much of a difference when I compare my old Olympus M43 setup to my current Fuji X ones. Haven't really experienced any portability issues (also tend to carry the camera + 3 lenses when traveling), at the same time I do not use long lenses so I can agree with you there might be size advantages for those cases.