Lenses for rangefinder cameras (e.g., Leica M), especially wide-angle lenses, can be more compact than those for SLRs—provided one accepts that peripheral rays strike the image sensor at an angle. With film, there were no problems. With sensor microlenses, things are more complicated—so much so that Leica sensors, to ensure good image quality from M-series lenses even with digital cameras, have thinner sensor filter stacks and microlenses much steeper, thus directing light rays more vertically toward a much closer photodiode, and/or different microlenses at the center and edges of the sensor.
As for autofocus (AF), the designer must adapt the optical design with a lens group that can move to ensure fast response times. This constraint is entirely absent in manual focus (MF) lenses, which can therefore be more compact (less elements/groups) than a similar AF lens. In fact, to ensure similar performance when an AF group is included in the design, the designer could be forced to add further elements/groups, and the optical scheme becomes even more complex (making the lens larger and heavier).
I also get the impression that using higher-quality (and way more expensive) glass could allow for simpler optical designs and even better overall performance. Leica lenses probably use this approach as well.
