I’m sure we can resolve this issue. Probably Damien will come around and tell us what we need to know, but I have found a couple of ideas that we might end up trying:
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Force a one way sync to AnkiWeb (choose upload). Damien suggested it here: Size of my collections - #9 by dae
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I understand that review history of deleted cards is still retained so that you have accurate stats. It looks like you can get around that by exporting to a new profile. I learned this here:
I have 10 cards in collection, but the collection size is 300MB
There is no harm in trying idea # 1 (make sure everything is in sync, and then force a one-way sync from the device that you are sure is up-to-date and choose “upload”), but I would wait a bit before trying the second idea to see if we get better guidance. As you said, so far it does not seem that review history is the main problem.