I think the issue for me is less the city size and more that it seems impossible to make a self-sufficient city. A lot of that comes from the fact that the cities are so small though, since every city works building comes at the sacrifice of valuable real estate. It's nice that you can upgrade buildings, but it doesn't cut it. I had a recycling center and a garbage dump, both with the max number of trucks, and yet there was still large portions of my 250k pop city not being covered, so I had to plop yet another one down. What's the implication here? I need a neighboring city that focuses on waste to help me out? That's fine for a solo region, but in a multiplayer set-up, who wants to be the guy who says "No, you guys build a cool city. I'll clean up your shit"?
If these buildings, fully upgraded, scaled with a high pop city, the size issue would be irreverent. Instead I am consistently pushed up against a wall where the only clear solution is to make another city. And since what the other cities need only surfaces if you specifically go look at them, the inter-connectivity that they are forcing is ambiguous and annoying.
edit: here's a shot