I'm not sure at what scale this makes a difference, but South Korea has a population somewhat larger than California in a landmass a little bigger than Indiana. The US constitution obviously wasn't designed with 300M people and 50 states in-mind, but it's larger and more diverse than South Korea, so it probably needs to be governed differently. It's like how having states in Singapore would be silly, just at a different scale. I do think what system of elected government makes the most sense for a given country based on size, population density, population concentration, and other factors is actually an interesting question.