It is not fair to use MICRO 4/3 as the size comparision. I am talking about APS-C or even FF in my suggestionThe reality is that with the 'pancake lens" the size should be the same as other mirrorless with the lens mounted.
On a micro 4/3 camera, the flange distance is 20mm, so with a pancake lens you have 20mm + the depth of the pancake lens (20mm for the Panasonic 14mm pancake). Flange distance for most SLRs is over 40mm, so regardless of how thin the pancake is, you get a larger package.
The idea and physical laws are the same, regardless of sensor size.
If you extend the logic, it applies to the NEX system too. Why the NEX cameras can be so thin/slim ? Because of their lens flange distance. The rumoured mirrorless lenses from Canon have a shorter back focus plane distance than the Sony ones. From previous CR citations, supposedly only 1mm distance! So you can imagine how close the flange distances are.
Also, part of the reason with closer flange distances is that it allows the design and use of smaller optics for an equivalent focal plane area of focus.