To get an understanding of the core database architecture, you can perform a test installation and compare your database to that. I'm afraid, we do not have a listing of current tables.
I say "core", as if you are utilizing any third party add-ons, these could mean that you have more tables than just the core.
While you did not explicitly ask this, we would never recommend deleting actual data, like forum posts for an example, in a case where your database is growing outside of your hosting's limit. Often this comes up from clients and it puts you in a situation where you will be forever chasing your tail and potentially compromising your community. For that reason, we always recommend moving plans or providers (if you can).