Sony, Panasonic are down to 60nm technology and the APS size are right now at 90nm from this two companies and Canon use 180nm at APS size but often 250nm". Canon is producing small sensors for one line of cameras (S100) with its 180 nm line.
Sony and Nikon full frame sensors, D800, are "still only 180 nm technology" vs all Canon's full-frame DSLRs (since 1Ds CMOS production in 2002) have been made with 500 nm technology.
Canon has today capacity to make small sensors with smaller cells = what they use in the compact cameras but no equipment to do the same with larger sensors surface like APS ,24x36, if they today shall make a larger sensor with high resolution its means stitching together small sensor areas to one bigger and that is expensive and slow process compared to equipment's who can handle and expose a larger sensor area in one exposure and also at the same time go down in smaller cell sizes.
Canon has with other words no equipment with such accuracy that they can produce larger sensors in one piece and with smaller cells= higher resolution with out heavily costs. Thats one of the reason why they have stopped at 20-21 Mp today and 24x36mm sensors.