It’s size and weight without the mirror box, so when I look at an image in my electronic view finder I’m seeing what the sensor is processing, with exposure ect.
On a DSLR you are looking through mirror box and seeing a reflected imagine off the mirror so you can’t see how the image will look until after you take the pic (most top range DSLRs now give you the choice of both views)
Also without the mirror box the lens can be closer to the sensor saving more weight which is why you see new lenses for Mirrorless.
There are various other differences but in terms of image quality there are the same.