Small update:
I completely erased the contents of 1 field out of 8000+ older cards (I no longer use that field for mature cards, only when they’re fresh) which was the potentially heaviest of them all, and noticed pretty much 0 difference in collection size.
I then went on to make an Anki collection package backup (without media) of all my current decks, and the final file size is: 19.6MB. (the compressed/uncompressed question applies here as well. I assume this is compressed size?)
Pretty heavy, but not anywhere close to the 91.30MB which I still have on Ankiweb.
Something is just weird. It can’t be that the reviews history takes that much size, since garrettm30’s comment contradicts that, but at the same time what else could it be? Does Anki keep a history of all the changes you make? Or maybe the “find and replace” ones? Because I did some mass changes via regexps on 10+ thousands of cards, multiple times, totaling maybe 40k changes. What about the scheduling? I have an addon that I use once in a blue moon that changes the scheduling of cards (taking a vacation/skipping a day/optimizing the scheduling of future cards so as to not get days with a bunch of cards/etc.). How about change of ease value via addons? Is any of that archived in Anki?