Trying to rebuild xapian indexes, indexer is indexing more messages than exist

I'm using Cyrus IMAP packages from CentOS stream. Pretty standard installation of the RPMs. Trying to rebuild the xapian indexes for a mailbox.

I do this:

rm /var/spool/imap/r/user/<username>/xapian/*
sudo -u cyrus -H squatter user.<username> >| /tmp/squatter.log 2>&1

The mailbox in question has only 1,663 messages in it:

# find /var/spool/imap/r/user/<username> -name '*.' | wc -l
1663

The entire mailbox, i.e., /var/spool/imap/r/user/<username>, takes up only 157MB.

Within a few minutes of starting the squatter command shown above, the /var/spool/imap/r/user/<username>/xapian directory is taking up more than 1GB, i.e., many times the entire mailbox size before I started the squatter command.

The squatter log file shows this over and over:

cyrus/squatter[1044372]: indexing mailbox user.<username>...
cyrus/squatter[1044372]: search_update_mailbox batching 20 messages to user.<username>
cyrus/squatter[1044372]: Xapian committed 58 updates in 0.009097 sec

If I multiply 20 by the number of "batching 20 messages" lines in the log, the total is many times more than the total number of messages in the mailbox.

What's going on here and how do I fix it?
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