My site has just shy of 2M posts from over 20 years. The post table contains 1.9G worth of data. Basically for every million posts, it is about 1GB of database space for the post table itself. The entire database is 6.5GB... including the conversion mappings for Xenforo -> IPB, keeping a permanent edit log, IP log, etc.
Another site has similar metrics in terms of post to storage ratio.
Now... attachments, avatars, etc are a whole different story. 😄