I think it's more than the size of the body. DSLRs have a professional aesthetic that mirrorless cameras don't. They are also better suited to the point-n-shoot style of shooting that most people are familiar with. Current manifestations of mirrorless cameras are a great way to introduce higher quality interchangeable lens systems to people who previously were not interested in SLRs, for some of the reasons I mentioned (and others I did not). Sure, the size fad will wear off but, that form factor is here to stay.