The topic_id and post_id fields are specified as UINT in the schema file, this results in a mediumint(8) unsigned in MySQL. This is too small for very large forums. Our forum surpasses 18million posts, while mediumint would cap out at 16m.

With current hardware there is really no good reason to use mediumint(8) over int(10), the amount of space saved is negligible for the majority of use cases.