That helps a lot thanks. Will summarize it quickly for those who come later: MySQL (InnoDB really) and Postgres both use MVCC, so they write a new row on update. InnoDB however also additionally writes a record marking the old row for deletion.

To do a cleanup, InnoDB uses the records it kept to delete old data, while Postgres must do a scan. So InnoDB pays a record-keeping price as part of the update that makes it easier to clear data, while Postgres decides to pay this price of occasional scanning.