There are two tables which can have immense sizes - buildroot_listing and tasks. They can be partitioned in case, there is sufficiently new postgres. Anyway, keep the option to not use it, if pg is older.
PR #1823
Metadata Update from @tkopecek: - Custom field Size adjusted to None
Commit 8cca8532 fixes this issue
Commit c6d1f347 fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1746
Please continue any further discussion there.