the steps mentioned above by Patrick are referring to PRTG 6, not 13. You can change the Data Purging Limits on the "System & Website"-Settings Page (https://www.paessler.com/manuals/prtg/system_website_settings.htm) in PRTG. The Historic Sensor data, which probably takes the most disk space is set to 365 days by default.
If you want to check PRTG's datafolder, to see which type of data takes most space, please have a look at the following article explaining the details:
Also, enabling the NTFS-Compression usually helps a lot on the PRTG datafolder, but sometimes has slight performance impacts. It's impossible to predict the impact, this is basically a matter to try out by the user.