The mirror less Sony Olympus have the same size sensors as the consumer SLR's they make take. In addition they can more or less take the same glass with an adapter. It is a massive benefit being so much smaller, my Olympus Pen EPL2 fits in a pocket, especially with the 21mm pancake lens.. The end result is a camera that leaves the house more often. As for paying a little more for an SLR, well they cost about the same because they only give up the mirror, ie doesn't cost less to manufacture.