All I can suggest if it is getting full that quickly, it sounds like maybe that you may have a lot of web crawlers indexing your site, so it is producing / creating a cache of all the elements thus filling it up so quickly .
Ensure that your site is running the latest grav version and up to date,
Ensure your site permissions are set up correctly on your web server.
Is there any plugin that you are using to provide search / indexing of the site which could be causing it ?
check your php settings for accelerators and caches settings.
Performance & Caching | Grav Documentation (getgrav.org)
Scheduler | Grav Documentation (getgrav.org)
Specifically this area
https://learn.getgrav.org/17/advanced/scheduler#grav-system-jobs
cache-purge / cache-clear
Also check if you have automated backups running , which will fill up the disk quickly
Also your hoster maybe to help to purge the cache or at least set a limit on the directory on the data size, so when its reached, it will basically overwrite the oldest elements in the cache .
try and block bots which are going to be scanning the site regularly , for good or bad purposes .. and check your .htaccess / web server logs and try and use settings to block anything that looks untoward.
:)