For once I will kind of stick up for SC2013 here, sort of. One of the reasons I think you will find SC2013 cities are so small is because of course the same engine has to cope with the online multiplayer mode as well. I've not done multiplayer in SC2013 but I'd imagine if you have 2 or more cities being built and simulated at the same time it means you have to cut down on the amount of sim being done for each individual city, hence they're smaller.
I understand there is always a compromise and you cant have it both ways with huge cities, with huge populations and also simulate all the things people do in your city....certainly not on the scale that so many people seem to want.....eg I want all the stats and tracking info of Tropico, bundled with the huge populations of SimCity, and with huge sized maps that allow for multiple cities and multiplayer on top of it. Its just way too much for any PC to handle.
Personally I feel C:SL is going to be a really nice compromise between all those extremes. Its offering a decent sized map that will allow for large metroplis cities, or smaller more specialized cities, and will sim up to around 1 million real cims/sims without any fudging of the numbers. I really dont think this game would work that well if an online multiplayer mode was included, I'm glad its single player only because it suites what this game is about....which is the ability to create your own personalized map, a city to a similar scale of the old SimCity games, and most people really don't want that city messed with by someone else in multiplayer. The only exception to that may be coop play where you both/all work on the one city at a time.