Yes, it sounds like you could have a lot of replication messages queued up to send to the other cluster member. How long has the other cluster member been decomissioned? Did you install and run the pgpSyncUsers utility on the server before decommisioning the other cluster member or attempting to upgrade?
Also, I would run a disk usage command on a few key areas of your server to see where the space is being taken up at (this needs to be ran through SSH obviously):
du -h /var/log
du -h /var/lib/pgsql/data
The first entry is showing how large the log files are (the bottom entry should show a total of all the folders/files in that /var/log directory)
the second one is telling us how big your database flat files are
If the data is in the database, there are additional queries that you can run to check which table is taking up the most space. But often times, it's web messenger messages. If you use the web messenger feature, this takes up space on your server and will cause your database backups to grow over time.