couple things to understand here...
alexa ranks sites overall. traffic, posts, users, etc. best to use this for overall and good information on the site.
big board rankings has been around a long time and ranks the top FORUM boards. it pretty accurate. it takes account total posts overall and I believe they have filter options now. haven't used it in a long time.
gaia online is a long time running anime website.
second place goes to 4chan which will probably surpass gaia at any point.
bodybuilding is also definitely in the top 20. the bodybuilding misc section and nutrition forums are some of the most popular sub-boards on the internet.