While, revlogs are a part of the total database size, backup files are not (and they aren’t synced).
No, that wouldn’t be the source of a problem. Rescheduling through FSRS Helper doesn’t increase the size of your revlog – but reschedule-on-change (in Options) does. Which one have you been using?
Where exactly are you looking to get each of those figures? To get the full total from “Note Size” – the easiest place to look is the bottom of your main Decks list. That will give you a “Collection” total, and if you hover over that, you’ll also get what portion of that is your revlog. If you’re looking somewhere else, Note Size might be giving you only the total related to a specific deck, or just to your notes, not your revlog.
That’s likely because downgrading Anki doesn’t also downgrade your add-ons. You may need to delete the add-on and reinstall it to get the correct version.