My general rule of thumb is that you should back up the log AT LEAST every half hour. If the database is busy, then do it more frequently. The busier the database is, the more frequently you should back it up. This is the corner stone of log file maintenance.
Database is not that busy.. There was no any such huge transaction as such ?
...OR how can I check if there was any such transaction which caused the log file size growth ?
When you shrank the log file on the principal, did it shrink on the mirror as well? If not, what build of SQL Server are you on?
No.. Actualy the log files size were diff.. think it was not synchronised that time. After we shrank and reconfgiured both are of same size now.
Thanks a lot to all.