Size of Log Backup is not reducing from past three days
Thank you markp, this was spot-on. There was a hanging transaction from past 3 days and that was the cause of this increment in log-size. 



After investigation, we found out that there is flaw in application wherein application doesn't disconnect itself if user doesn't close them. We called user and once he closed his session, everything started smoothly and logisze came down from 9GB to 300MB.

I am posting this here as an answer, it could helps others in case they face similar issues. Thanks once again.