I agree in theory, but big cities are where things get muddy.

When a single city (e.g. New York City, population ~8 million just to use the biggest example) has a population larger than entire states, how do you "split" the state of New York? If the city itself, excluding any of the surrounding "metro area", was its own state, it would be the 13th most populous in the US and also the smallest by area.

Do we carve up each of the boroughs as a separate state, and give New York City 10 senators? It would be more proportional representation for the people of NYC, but also their close proximity and interdependence would very much align their priorities and make them a formidable voting bloc. And even then, you could still fit 4 Vermonts worth of people into Brooklyn alone. How much would we need to cut to make it equitable? Or do we work the other way as well and tell Vermont it no longer gets to be its own state because there aren't enough people?

For states like California, which still have large cities but not quite to the extreme of New York, how do we divide things fairly? Do we take a ruler and cut it into neat thirds, trying to leave some cities as the nucleus of each new state? Or do we end up with the state of California (area mostly unchanged), the state of Los Angeles, and the state of The Bay Area?