People who have never been to L.A. really have no idea how insanely huge it is. Driving to my apartment from the start of city (before you even get to L.A. county) and having the city just keep going and going and going for two hours and not because of traffic jams is something you have to experience to truly understand.
Paris is quite insane too, smaller but with an insane number of inhabitants per square km. Their metro isn't as clean but ut shuffles people around for sure, 650.000 passenger per day for metro number 13 for example. Or so it was when I lived there.