Whole list can be seen
here.
I'm sure my country will be in the top 20 on the list if it's extended, except if offline gambling isn't counted as part of the stats. The rate at which lotto kiosks and centres spring up in my country is really alarming and it goes to show how addicted youths are becoming to gambling. As for the US topping the list, I ain't surprised. The US is a country that doesn't restrict stuff which keeps citizens busy and entertained, provided those who run them remit taxes to government.
I think so too mate, in my country I normally see people gamble offline than online, although we are not there to know if they are also gambling online or just offline. However, I actually think all this are just based on statistics because despite the fact that some countries prohibited gamble but I am also still sure that some of them are still using vpn to gamble. you are right, US is a popular country and am sure that they have more gamblers than other countries.