Chicago has to be short to make the tight turns built into the system (especially some of the elevated lines). The height/width of the cars seem pretty normal to me (unlike London, which clearly is smaller to fit the tunnels).

Funny, though - after spending time in Chicago, once I get back to DC the metro cars seem to be about a mile long, and actually THAT kind of bothers me more. Also worth noting that here, because of the length of the cars, there's a perpetual problem with people bunching around the doors - a fear by many, I guess, that they'll get jammed into the interior of the car and not be able to get out in time once it gets to their stop .