[Brightcolours]

It is a lens for 18mm flange distance. That means it can be made small. For DSLRs. 21mm needs to be a (more?) retrofocus design, because of the bigger flange distance. It is only 21mm and f2.8, this lens is not remarkably small after all. They can be much smaller, but Zeiss lately is designing all kinds of corrections in their lenses, so they become complex with a lot of elements, and I think that forces this lens to be a retrofocus design as well.

 

For real compactness, look at the Voigtlander 20mm f4:

http://voigtlaender.com/21-mm-f-4%2c0-color-skopar-p-typ.html

The pancake 20mm for SLRs is very compact too, by the way.