After looking up things, I guess I wasn't as mistaken as you made me out to be. True, the size (as in land) may be the same, but the size (as in population) isn't even close. I googled "max population in simcity 3000" and "max population in simcity 4." In Simcity 3000, the average seems to be around 2.5 million while some have popped the 10 million mark. In Simcity 4, the highest I saw was over 1.5 million. People consistently build in the 600,000-700,000 range. Big difference in size.
Bzzt! Wrong again
http://www.youtube.com/watch?v=nuu943ieuRIPopulation in a MEDIUM tile (that's 1/4 of the SC4 Large or the SimCity 3000 tile): 3'242,424
No population mods were used.
http://www.youtube.com/watch?v=YF_hM1Wpkzk8'340,902 (don't know which terrain, so I'm assuming the biggest). Money cheat, but no mods.
People usually don't make their cities that big because they play around in many cities.
In SimCity 3000 you played one city until you were bored with it and never touched it again, so yeah you reached a higher pop.
In SC4 you can have a RIDICULOUSLY DENSE city if you spend a lot of time making people commute to work to another city. Or having the entire city be super-hightech industrial with nothing but the biggest $$$ residentials, because all your pollution and "plebs" are also in the next city.
And as 10ebbor10 said, the population is just a number, they could have used 10x cap in every building when they made the game, make transit have 10x the cap, make everything 10x as expensive (so the increase in taxes would be irrelevant) and the game would have been exactly the same as it is now.
It's a bit like saying that chars in Final Fantasy are stronger than D&D ones because they can have Strength 400 instead of 18.
I think when people complain about SC5 city size they don't just talk about the number in the corner of their screens, but about the actual complexity and amount of buildings they can have, so yeah, they're talking about terrain size, and how many "agents" you can have running around before the processor grinds to a halt.
And remember: In SimCity 2000 you could have "infinite" population.