League of Legends has about an order of magnitude greater number of players. However, Dota 2 has until recently been in open beta, and League of Legends came out years earlier. Although the beta keys are essentially obtainable for anyone, the extra step in obtaining said keys may be enough of a disincentive for many people to not play.
Source for LoL numbers: http://www.riotgames.com/articles/20130312/700/league-legend...