Using a Sony A7iii and a 24-70mm lens - fits well (a bit tight - a needs some patience to put on properly). The little mesh storage is not as packable as I would have hoped, but for the peace of mind it's well worth it! The corner clips are a bit difficult to get through the shell but once in they are locked and loaded.
My opinions of shell fit for my mirrorless Olympus (OM Systems) OM-1:
For the OM-1 with the 12-40mm Pro f2.8, the small fits and can cinch be cinched around the lens. But the zoom ring is close to the base of the lens and can not be used with the shell on, so I have to pull shell off to zoom. The medium fits just as well, and since I have to pull shell off anyway to zoom, the extra fabric is irrelevant and there is no need to cinch around the lens.
For the OM-1 with a prime lens smaller than 75mm, the small is too big to use if you want to cinch the shell around the lens. If you want to just use as a cover while hiking and pull cover off to use, then small is fine. Both a small and medium will cinch around the 75mm prime, so both work fine for the 75mm.
For the OM-1 with the 40-150 Pro f2.8, the small does not full cover the lens. The medium fits perfect, and the zoom ring can still be used with the shell cinched. The zoom ring is closer to the end of the lens than the 12-40mm, so the slack in the medium shell fabric still allows the ring to be turned.
Due to the limited fit and usefulness of the small shell, I recommend the MEDIUM for the OM-1.