Most hosting control panels have a function to see disk space usage. The main one that I work with is cPanel, and I know that ha sit. If you use cPanel, you can see which folders contain the highest file sizes.
In cases like this I would start looking at files that are not part of wordpress unless you have been uploading videos or very large image files. I rememeber one site a while ago that had an error log file that was over 1.4GB by itself, so add in a few of those around the site, andit's easy to get up to 10 or 20GB quickly.
Also, remember that on most hosting accounts all of the sites files count towards your storage - and this includes emails, logs and anything else that's associated with your site. A lot of people store all of their email on the server and that takes up a lot of space as well.