Its looking at the biggest stadiums (or stadia if you like) in the world, specifically those with a capacity of at least 60,000. You can hover and filter to select different sports and regions, and a picture of each stadium should pop up.
1. College football stadiums in America are HUGE. They make the NFL stadiums look tiny. Think about this for a second - these are teams run by institutions of higher education with unpaid players and there are some 46 teams with seating for over 60,000 people. The biggest, Michigan Stadium, is the third biggest stadium in the world. The Premier League by comparison has 2 teams that meet this criteria.
2. I was surprised by the lack of baseball stadiums. I guess it has to do with the shape and size of the field. The biggest baseball stadium is Dodger Stadium with a capacity for 56,000.
3. There are big stadiums really all over the world. There are loads in Africa and Asia, and pretty much everywhere. There are definite regional differences between the sports enjoyed, and these become obvious in the map. The UK is probably the country with the greatest diversity of sporting venues at this level.
4. I'd quite like to go to the 'Mass Games' in North Korea, its probably quite a sight!
NB - I lumped Rugby Union and League together, sorry if that bothers anyone. Also I tried to pick the 'primary' sport for each venue but may have got this slightly wrong, if you spot a mistake, please let me know.