Yeah but maybe that's a good thing. Otherwise you have a situation like New York where NYC gets to dictate everything since it's super high population density even though the majority of geographical NY state is farmers. So the farmers get screwed over by the interests of the city folk who have never even stepped foot out of the city. For example, city folks are more likely to vote for increases in property taxes - not that they care too much since they only have a tiny apartment - but it hits the people outside of the city with lots of land (for farming) really hard.
It's just like the "rich getting richer" conundrum. Once you have a large city, it will gain more people faster than a small town just by virtue of the properties of growth. So pretty soon you'll have a few megacities that get to dictate the government of the entire country and if you live anywhere else; too bad, the city folk are in charge now.