Three times is the upper limit (from my understanding) as there is the snapshot copy of the fishbucket and a temporary copy.
You can lower:
[inputproc]
file_tracking_db_threshold_mb = 500
However you must be aware that lowering this value may result in files been re-indexed if they are still on the filesystem (500MB is a very large fishbucket so the chance of this happening is super-low). For example if the fishbucket had tracked 2 million files, and you still had 2 million files on the filesystem, if you reduced the fishbucket size to only track 1 million, then the remaining 1 million will be re-indexed again.
I've lowered it before and had minimal issue except on servers with hundreds (or thousands) of old files, after a few months the files would be re-indexed as they were the oldest entries in the fishbucket which were removed and then the file got re-indexed as they were still in the monitored directory path.
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