This seems pretty obvious to me (as a european resident). The amount of stuff you have to figure out to reach the US market is a lot less than the amount of stuff you have to figure out to reach the european market. That's a lot of the impetus for tighter european economic integration. If the cost of reaching the US market is a fraction of the cost of reaching the combined european market, it doesn't matter if there are a few more people in the EU. You also can't really measure online market size by GDP or pure head count. Some corners of the EU (Bulgaria, for instance) are less developed than Mexico.