I agree the sheer size and population of the US accounts for it's relative isolationism and introspection.
Watching the news in LA I found most of the TV news covered events in California and then when it went to "Now for the rest of the news" it went to New York.
Here in Ireland with a small land area and a population of only 4 million by necessity the news spends more time on international items simply because there are insufficient local items of interest to fill the news slot as Mrs Murphy's dog is missing is unlikely to grip the audience
Another aspect is the power ratio. The more powerful a country is the less bothered it's citizens are in what other countries think of them and so for example I would think the average Iranian citizen is extremely interested in anything the US says about them to glean some insight as to whether an attack is coming (newsworthy) whereas the average US citizen secure behind their military might has no such worries and so is correspondingly less interested in whatever emanates from Iran (not newsworthy).
So I wouldn't say Americans are dumber than their international counterparts because they are less familiar with world geography, it is just their focus is different.