I'm fairly pleased with the size of my Panasonic GH3, but I won't use the 35-100mm f/2.0 on it, although I use the 14-35mm f/2.0 and 50-200mm f/2.8-3.5 frequently. Of course, compared to the E-5, the GH3 seems small.
External size:
Width x Height x Depth
E-M1 130.4x93.5x63.1
GH3 132.9x93.4x82.0
E-1 141x104x81
E-5 142.5x116.5x74.5
Weight:
E-M1 443g
GH3 470g
E-1 660g
E-5 892g
The E-M1 is close to the GH3, except at the grip.
not to forget space is needed to house an improved 5-axis IS system; the other camps do not have this IS.