Total population statistics are pretty worthless to determine players of this game... I would bet the players per capita is very different.
US population~319 million people
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As we build more compelling tournaments like the System Open Series...
You mean the "compelling tournament" that only had a single event in the US, aka your biggest player base, and completely disregarded the Asia-Pacific region entirely.
Forgive me if I don't accept that as a decent line of reasoning to explain the inclusion of IDs....
European Union population~508 million people
Tell me US has the biggest player base again please
US Open - Allows for 265 people
EU: UK Open alone allows for 512 people. You also have the France Open, Poland Open, Denmark Open, Italy Open, Spain Open, and Germany Open
So, yes, population does not equal players, but I would ask you why they have so many more EU Opens with much more people when the EU is smaller in geographical size?
The EU Opens likely have more players given the ease in which one can travel between multiple European countries both as far as distance and cost are concerned. It is entirely feasible for some in the UK to also attend the French and German opens without too much trouble or cost.
This is opposed to the US, which given the geographic size and cost of travel, makes it unlikely for people to travel across country to attend events.
From what I've heard Americans travel further than Europeans for tournaments.
Some americans maybe, but certainly not all. I'd LOVE to go to nationals, or worlds, or the system open, etc. Except I live in Alaska, so it's 3000 miles away. Acording to [here](http://www.worldatlas.com/webimage/countrys/eulandst.htm), all of europe is only 2000 miles vertically and 1339 miles horizontally.