Originally posted by Uluru
As a comparison: Sigma 18-35 is wider (78mm) than DA 20-40's overall height (65mm).
Good point indeed. The Sigma 18-35 is way, way bigger than the Pentax 20-40 (which is to be expected with a lens with such a fast aperture).
Making a lens smaller than normal usually means a compromise on something else, either image quality, focal length range or maximum aperture. Ricoh-Pentax did a great job in keeping the size down with the DA 20-40mm, making it a great travel & street photography lens. Great to capture stills and videos of shy people, whereas a big lens will usually scared them away.
The drawback are the compromises, the first being the maximum aperture. While F2.8-4 isn't bad at all, it's far from what the Sigma 18-35mm F/1.8 can offer in terms of shallow depth of field control. But that thin DOF comes at a price, hence the huge size of the lens. The other compromise could be image quality. Sigma managed to keep the IQ very high on its 18-35mm F1.8, but they had to make a compromise on size to achieve that.
Most probably, the compromises Ricoh had to make on the 20-40mm F2.8-4 didn't affect image quality. They already made those compromises on focal length range (a 2X zoom isn't very impressive on that aspect,

) and on the maximum aperture, which is good, but not very fast by today's standards for such a short range zoom. So the IQ should be quite high in the end.
Kudos on the good job, Ricoh!