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 ?