Leica 35 mm lenses- small and powerful?

Have recently visited the Leica store in Washington DC, I think it is the first in NA (was very surprised). I played a bit with range finders as I have never had this experience and talked to the guy who is using the, in teh store (he told me that he still misses focus quite frequently). What realy surprised me is thesize of the 35 f/1.4 lens....so small, incredibly small and I started wondering why other 35 mm lenses are so latge and heavy? Take Zeiss, Canon, Nikon....
I understan that the camera is a different technology, but how they could produce such a stunning lens that is so small and light? Why other manufacturers are not doing it?

Rangefinders lenses can be made smaller because they are closer to the film plane or image sensor.

http://en.wikipedia.or​g/wiki/Rangefinder_cam​era

There are two reasons I can think of. The first is quite obvious - the lens is manual focus and manual aperture, so no requirement for the space that electronic control of those requires. The other is perhaps less obvious, and it is that the Leica rangefinder format uses a shorter "registration distance" which is the distance from the lens mmount to the sensor/film plane. SLR lenses shorter than 50mm or so have to be designed as a sort of reverse telephoto, to project the image back to the sensor a further distance than the focal length so that there's room for the mirror to lift. This requires more glass, and bigger front elements. For the Leica rangefinder this is only true for lenses shorter than about 35mm.

Pentax has a series of well regarded lenses...the Limited series. These lenses are small and provide excellent photos. They are designed for Pentax DSLR's.