One of the advantages of mirror less cameras is that there is no need for the space for the flapping mirror and pentaprism. This means a smaller flange distance so retro focus lens are not required (though most modern lens are), so lens can be smaller and less complex but give potentially better image quality. No moving mirror means less likelyhood of camera shake, plus without the hump on top housing the pentaprism the camera can be smaller.
The camera just above, the Nikon DF is about as small as you can make a full frame digital SLR with traditional optical viewfinder because of the need for the mirror and pentaprism housing. So why with the current batch of mirror less cameras are companies like Panasonic, Olympus and Fuji sticking a fake hump on top making the cameras bigger, is it hump envy, do they feel the need to look like ‘real’ cameras or is there a valid design reason?
By the way if you want to see a ‘real’ camera check out my Ebony below:
