In mirroring the log cannot be reused until a log backup occurs.

My suggestion is that if the log is growing to large between your log backup job executions then you should increase the frequency of the log backup job.

I had to do this for one of my servers because for one database through out the day the log would grow to 60GB and I was only taking a backup once a day. i modified this to happen 6 times a day and now the log is stable at about 20GB.