I am not familiar with the lens system used by the A7, so keep that in mind in what follows.
The reason why we expect mirrorless cameras to be smaller can be attributed to the distance between the rear lens element and the sensor (flange focal distance, roughly) potentially being shorter than that of DSLRs. DSLRs flange focal distance is governed by the mirror box (so that the mirror does not strike the lens while flipping up).
So if you design a new lens mount along with your mirrorless body, you may be able to exploit a shorter flange focal distance to make the body thinner. Not sure how this will impact lens designs (in terms of size/weight) in general, but I suspect some wide-angle lenses can be built without the retrofocal group if the flange focal distance is shorter.
Unfortunately, the shorter flange focal distance also means the light will hit the edge of the sensor at a shallower angle, which is undesirable in terms of microlens design and cross-talk (i.e. colour bleeding in the corners unrelated to CA). But Sony is working on a curved sensor which should (in theory) remove this hurdle.
And then you have to consider legacy lenses designed for DSLRs. You should always be able to build an adaptor to make them work with a mirrorless body (with a shorter flange focal distance), but adding an adaptor negates the benefit of the thinner body, not to mention the slight tilt it may introduce to the lens mount, further ruining image quality.
So it actually makes sense to build a mirrorless body that retains an existing lens mount (designed for DSLRs), unless you are also building a complete set of lenses for your new mount too.
But then your mirrorless body will be more-or-less the same thickness as DSLRs, although it should shed a few grams in weight (no pentaprism in viewfinder, no mirrorbox). Not sure what route Sony chose for the A7.
