Do you know the table which is taking most of the space?
Updating retention only applies to workflow executions which closes after the retention is updated. All workflow executions closed previously still need to wait on the timer based on previous retention policy for them to be deleted.
Another cause could be that we create a 10 year timer for each workflow timeout and potentially activity execution. Although these timers are tiny but they can add up if you are starting workflows/activities at a very high rate. We have a PR out to address this issue. Should be included in our next release.