David and Bernard,
There is another way to interpret the same facts! Sony, Nikon and everyone else started in a more mainstream "APS-C" format, launched an array of lenses for it, and then added the larger 36x24mm format (Sony did this twice: first with SLRs/SLTs, then with mirrorless). So for obvious reasons, with the larger format system arriving later, it needs to catch up on lens offerings, while there is less need to add lenses to the earlier, more mature "APS-C" lens system. There is also the fact that many lenses for the larger format are also perfectly suitable with smaller format bodies, such as primes about 60mm and up, so many lenses double count for both formats. Also, there is no question that the larger format calls for more high quality lenses than the smaller format, and its users will eventually demand a wider range of lenses than the more "entry level" smaller format cameras—at least once a camera maker deprecates the smaller format to a lower quality tier, as Nikon for example has done to Thom Hogan's chagrin. (On the other hand, Olympus, Panasonic and Fujifilm are still active in providing high quality "enthusiast to professional" quality bodies and lenses for their smaller format systems. A famous quote: "to succeed with Plan A, you must have no Plan B".)
Another way to measure the camera makers' interest in these two sensor formats is the number of bodies and frequency of their updates. For both Sony mirrorless and Nikon SLRs it is about equal, while Canon releases far more SLRs in its smaller EF-S format than in EF. (Maybe Sony and Nikon could take a tip from their more successful rival!)
Overall, I still see no resolution of the question as to which format is likely to contribute more to revenues and profits of a mirrorless camera system.