I'm normally quite stupid in regard to technical things, but doesn't matter what system it is a 20mm lens is a 20 mm lens. Makes no difference how the sensor sees it, so the physics are the same. Could be wrong, but just my punt.
Flange distance (mount to sensor) on a canon is 44mm (45.46 on pentax, 46.5 on nikon). So the 'easiest' lenses to design are in this ballpark, and why most pancakes are 35-50mm or so for dslrs (lots around 40mm, just shorter than the flange distance).
Longer Fmm than this and it becomes more and more telephoto (light-ray bending) to get the lens more pancaked. Shorter Fmm than this and you need to design more as retrofocus or reverse-tele (and the lens also gets longer).
4/3-mount is 38.67mm, so pancakes make most sense around 30-40mm.
Micro-4/3-mount is 19.25, so pancakes are easiest to design (and thinnest) at the 15-20mm length.
Because they both use the same 2x crop factor, a 4/3 pancake will be 60-80mm FF-equivalent-FOV, a m4/3 pancake will be 30-40mm FF-eFOV.