Once we start looking at the E-XX and E-X, as well as the SHG lenses, the whole argument about lighter and smaller is lost!
Well, compare the size of Oly's 300mm f/2.8 to Canon's 600mm f/2.8 and the argument is won...oh wait, there isn't a Canon 600mm f/2.8...well, compare it to Canon's 600mm slow f/4 lens then! The argument is still won!
As for body size/weight, that's a personal preference and is dependent on the user's hands. Smaller cameras may not be best if the photographer has large hands. Having tried the Micro FourThirds cameras they are just too small for (my) serious photography...and cameras like the E-1/E-3 are too big for me to tote around.