Log sizes unexpectedly large: log_reuse_wait_desc == "log_backup"
Few suggestions from my end, Kindly check and let us know


Run SQL Profiler and check for any unusual activities.
Check if the increase in size of the log files happens regulary at or around the same time (If yes, growht in log file might be the result of scheduled job)
Run DBCC LOGINFO  and check if the VLF's are active or inactive.
I randomly GUESS the result of the log growth is due to index maintenance activities you might have scheduled.
Lastly, Just for information: You need to change the database recovery model to SIMPLE and then perform the  DBCC SHRINKFILE  command.