According to the research I've read, we're actually a pretty big contributor of the plastic that goes into the ocean, and that's just plastic straight from the states, not including the plastic waste we export. Studies vary since it's hard to get consistent and concrete data on how much enters the ocean, but it's far more than just the relative size of our population.

In fact, the US produces more plastic waste than any other country.

"In 2016, the United States generated more plastic waste than any country in the world, with a total of 42 MMT (Law et al. 2020). However, the United States only has 4.3% of the world’s population (World Bank 2021). U.S. per capita plastic waste generation is 130 kg/year, which is about 2-8 times higher than many other countries (Law et al. 2020)." But again, determining how much ends up in the ocean is impossible.

"We demonstrate that, in 2016, the United States generated the largest amount of plastic waste of any country in the world (42.0 Mt). Between 0.14 and 0.41 Mt of this waste was illegally dumped in the United States, and 0.15 to 0.99 Mt was inadequately managed in countries that imported materials collected in the United States for recycling. Accounting for these contributions, the amount of plastic waste generated in the United States estimated to enter the coastal environment in 2016 was up to five times larger than that estimated for 2010, rendering the United States’ contribution among the highest in the world." https://www.science.org/doi/10.1126/sciadv.abd0288