The size theory does not hold water when you look at places like Australia and Canada, even New Zealand way down in the Southern ocean which is connected to everything by sea floor cabling has better retail speeds than most US cities. The major difference between the US and Oz/Europe is that the US ISP market is not a free market, it's a series of government granted monopolies which are by their nature are heavily weighted against consumer choice. Consumer choice is what drives competition in any market, the dismal state of retail internet in the US is entirely predictable and has nothing to do with an averaged US population density.
Trivia: Oz is ~0.8X the size of the US but the US has ~15X as many people. Canada is slightly larger than the US, shares the same continent with the US, has ~0.12X the US population, and yet both nations provide faster (average) speeds than the US.
The rest of the western world can clearly see that American's are getting screwed in broad daylight by telco's and health insurance companies because people like you still believe (and parrot) the ridiculous excuses these corporations and their pet poli-critters have to offer. Europeans are no more or less "geographically challenged" than Americans, rather your natural tribal instinct has been expertly manipulated* by corporate propaganda to fight against your own best interest. In other words we can see the root problems in the US and are trying to tell you there are better ways to structure the telco/health markets. It's you who does not understand what is happening to you and why.
manipulated* - We are all heavily influenced by what we read, hear and watch, the only effective defense is to practice self-skepticism, ie: don't just blindly accept one cherry-picked data point, pick your own comparisons and do some basic sanity tests on the "economies of scale" excuse.