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As you will notice in the 4 graphs, the A900 and E-M10II are pretty much equal in 3 of the graphs, but the E-M10II has much more dynamic range in the dynamic range graph. Also the E-M10II goes to a higher ISO. Although current FF and APS-C is even better I am pretty satisfied having gear the size/weight of m4/3 with an even better sensor than the FF one in the A900. I have read claims, but haven't checked, that the current 20mp sensors in the Olympus PEN-F and the Panasonic GX8 are a bit better than the 16mp.
Of course, some people want bigger gear. Certainly, if I worked in a studio or transported my gear around by car and then at a destination didn't venture far away for long then bigger/heavier gear would be okay, maybe even preferable. Not only do I travel and then spend many hours per day wandering around on foot, but even when I am not traveling I usually walk around for hours per day with my camera. In more familiar areas that I have wandered in before I usually just put a prime lens on the body and also remove the detachable grip. Usually I will have either the 20mm f1.7, 25mm f1.8, 45mm f1.8, or 14mm f2.5 on the camera in those cases.
Sony 28mm f2.8 attached and Panasonic 14mm f2.5 attached.