I did not convert from 7.9, I read this thread : Purging the config.idb after splitting frontend/backend and both PROJECT_CHANGES and PROJECT_RESOURCES tables are empty.
I don't, I removed all the tag providers and the size of the backup did not change.
I found that the culprit was the TAGCONFIG table. After pruning this table, the size of the config.idb went down from 1.8Go to 4Mo. Do you know the potential side effect of deleting this table ?