Amount of indexed pages

i would like to know what does Google or Yahoo take into consideration before indexing forums posts ... Some forums have many indexed posts in SE while some have a few only ..

Does it depends on backlinks or any other factors? i am a bit confused on this .
Blogs have posts indexed quickly in Search engines while for forums it is not like this ,

Forums generate far more content than a typical blog, therefore it is a more difficult task to get everything indexed. Larger forums, over time, should get indexed a bit better. But, to encouraging indexing and achieve better search engine positioning, it certainly requires a full range of SEO strategies, not limited to backlinks (but a very important part).