Simple OMD versus GH3.

That's a really good point and on top of that Panasonic realised that to make a full-featured true hybrid stills and video camera with all the expected pro features, that would not overheat when shooting long takes and that would be comfortable to shoot with all day, it would need to be a bit larger. I think they made the right decision and in doing so have produced a model that complements   the OMD rather than competing with it. It is a logical progression from the GH2 and does not mean that the other ranges (GF, GX and G) will inevitably adopt a larger format factor in future.