The top richest countries will probably always be very small ones with very special situations, such as Monaco and Luxembourg. Monaco is a tax-haven less than 2 square kilometers in area, inhabited by many extremely wealthy immigrants. Its GDP per head is about $70k. Luxembourg is a bit bigger, about 100 miles squared. It is also a tax haven, but also has a very large banking industry.
Besides these tiny tiny countries, there are Norway and Ireland, which both have about 4.5 million people. Small, but not ridiculously so. Norway is up there because it is sitting on more oil than any other European nation, and has a government that invests it rather well. Ireland has very low corporate tax rates, which have recently been attracting corporations to set up headquarters in Ireland, bringing in tons of new income and high-income citizens to Ireland.
In most cases, the richest countries in the world are relatively small and have special circumstances that allow wealth to be spread amongst relatively few people. Of the top 10 richest countries, 3 have less than 1m people, 5 have less than 10m people, and only 2 have over 10m people; The Netherlands and the USA. The netherlands has about 16.4m people, and the USA has about 300m people.
Among large countries, the U.S. dominates in GDP per-capita. The third most populous country in the world is consistently in the top 5 richest year-to-year. It is unique to have such wealth spread out amongst so many people. Comparing it to countries with between 100m and 500m people, it dominates. But those countries are Indonesia, Brazil, Pakistan, and other countries that are not at similar stages of economic development. A better comparison would be to Western European nations, Japan and Canada, which are all equally developed, but have circumstances and economic systems that vary.
According to the International Monetary Fund, the U.S. has a GDP per-capita of $43,000. Canada and Britain are the closest large nations, at about $35,000 each. Australia has $33,000. Japan, France, Germany and Italy all have about $31-32,000. These nations all have diverse economies with many different sectors, just like the U.S., and all are about 25% poorer than the U.S. is.