well, the size depends on the sensor size and the optics. Nobody beats physics. So it doesn’t matter whether we have a DSLR or a mirrorless cam: when both use a full frame sensor 24 X 36 mm, the standard lense is 45 mm. You may shorten slightly by new lense design the distance between sensor and centre of lens (in longitudinal sense of the sense, of course), but we’re talking about 2 to 4 mm which may be used to shorten the overall system. So instead of a 50 mm 1.4 DSLR lens, we may have a lense sized like a 47 mm 1.4 lens, but with 50 mm focus. So it isn’t such a huge difference.
Mirrorless apeared to be quite compact, as the first ones which somehow worked where the 4/3 ones, so quite a tiny sensor compared to APS or even FF