Everyday disk space on Exchange server keep getting low

It started week ago my exchange had crashed in the morning, event viewer says because of low disk space, everything was good before that, I endup restarting the server after some troubleshooting, everything came back as it is after restart I got 16GB free space,
but now everyday that free space on exchange drive is keep getting low, 16gb->13.5gb->11.1gb… now it is 5.5Gb, everyday full backup is successful, with no transaction log files pileup, the exchange data base is about 12gb and drive is 40Gb, in total the c drive should have only 21gb, I did the sizes with foldersize and diruse.exe
Please help me out