A larger circle indicates more hubs at that location.
Red circles indicate how many hubs are at that location using the coordinates that users provide in their profiles. This location does not necessarily represent where the hub actually is.
Green circles indicate how many hubs are in that country as indicated by the hub's IP address and mapped to a country using MaxMind's GeoIP. The coordinates of green circles were obtained from Freebase.com and do not seem to have a consistent meaning. Some are at the geographic center of a country while others appear to makr the capital of that country.