Why MSDB transaction log file grew while MSDB was in Simple recovery mode? I have Availability Group of 3 servers, and transaction log backups for databases in Availability Group are taken on a secondary replica, each 1 minute

It was working like this for several months


  Now MSDB on this secondary replica is 6 GB data file size (mdf) and is only 4% full and log file size is 14.2 GB (ldf) and is 7% full


I can only remember that I made MSDB 500 MB both data and log files in the beginning when server was setup...

I thought log file grew because of often transaction log backups and FULL recovery mode - but no, MSDB has SIMPLE recovery mode on that server

So it left me wondering, what caused both data and log file growth for MSDB ?
For me, large growths in MSDB are from adding and deleting the table entries within the backupset tables. Do you have a history purge or anything running?