The U.S. debt is big because the GDP of the U.S. is big. As you say, in percent terms it isn't so high.
The only big OECD countries better than the U.S. are Mexico and Turkey which are significantly poorer and so tax and spend less. Also, through NATO and specific alliances (ROK, Japan, Taiwan, ANZUS...) the U.S. defends those countries lowering their defence budgets in exchange for their own.
I would add immigration, but barring actual White (or Black because there are a few like that) supremacists who are a tiny minority, Americans are not opposed to immigration per se, even Trumpists which are the hardest line on that. The anti immigrant spiel of the anti immigrant parties of Europe (who are themselves a minority, and I'm not saying otherwise) would not be accepted in the American political system.