That's the excuse, but today's computers are so much more powerful than the ones we had when SC4 came out, it's really not a very good one. Gaming worlds should be getting larger, not smaller!
If you design your game well enough, a large map is always possible these days, unless you want to simulate every single blade of grass individually or some other such silly thing.
There seems to be such a huge disconnect between EA and what the simcity enthusiast community really wanted out of the new simcity. We wanted bigger map sizes, not smaller. We wanted the majority of our gaming to be single player, not multiplayer. These are very fundamental game dynamics that they got wrong, and mostly in the name of combating piracy. I'm against piracy myself, but as soon as you're going to start sacrificing gameplay for it, that's where I start questioning your tactics. EA has killed many a game studio and franchise, and simcity just seems like the latest victim. At this point supporting EA with my money in any way just seems.. evil.