DPReview forum member, viking79, has put up a photo-rich thread at DPReview comparing the Sony a7r to the Olympus OM SLR. As you can see, today's benchmark-creating 'small' is larger than yesteryear's compact. And with an NEX-OM adapter attached, the a7r becomes larger still.
The idea of the big-ass SLR is a myth.
While I dislike the Sony a7r for anything but work, I realise that it is a revolutionary product. It is redefining the expectations of photographers, who, for the last 18 years or so - and with the notable exception of cameras such as Nikon's FM3A - have expected quality 35mm interchangeable lens cameras to be huge. This was only exacerbated when cameras went digital. In fact, today, many Nikonians think the Df is small. It is not.
While the a7r isn't 'small' by comparisons to history, it is the first full-frame digital camera whose dimensions compare somewhat favourably to classical cameras. Of course, the OM is much better made than the a7r is, and has a much better finder, but at least the a7r points toward a smaller, brighter future.