From a source I find generally accurate: Compare camera dimensions side by side If you position the EM-1 in front of the GH3, the primary difference is the height of the camera's shoulders. The height at the hot shoe appears identical, and the EM-1 very slightly (2.5mm) narrower in width. Personally, I think the extra height of the shoulders on the GH3 is probably an advantage, as it allows for a taller grip area. Comparing depth or the body itself is more difficult, since the dimensions include the extension of the eyepiece and the grip. The GH3 appears to have a somewhat deeper grip, a slightly thicker body, and a slightly more protruding EVF, but none the differences seem huge.