GCP /dev/sda1/ Disk Space Full, Shows 90% In Use, but Mounting SDA1 only showing 6GB. What is using all my space?

I'm running Wordpress instance on top of a Linux GCP VM instance. In SSH, running cmd 'df -h' I see /dev/sda1 is 91% used up. I mounted sda1 to see what could be taking up all the space, but mounting it only shows 6GB. How can I find this other usage? Please help. I've keep expanding my Disk space on GCP. I started with the free tier 30GB, then pumped to 40GB. It keeps filling up and now I'm up to 50GB and I imagine it is only a matter of time till this fills up again. **UPDATE**: Seems I may have a mysql bin logging issue.