As you explore the Outerworlds Universe, the files and data which describe the appearance of the 3D worlds are downloaded from the Internet and stored on your hard disk. So that you don't have to keep re-downloading the same files over and over again on subsequent visits, they are saved in a folder named "cache" within your Outerworlds installation folder.
The more your explore Outerworlds, the more files are downloaded and stored in your cache. The size of the cache can grow to many megabytes after extended use.
By default, Outerworlds will not allow the size of the cache folder to grow past 100 megabytes. If this limit is reached, Outerworlds will start scanning the cache folder for old files to be deleted in order to make room for the new files that are being downloaded.
In the Disk Usage dialog, you can adjust the maximum size of the disk cache. You can even disable the cache size limit entirely by selecting No Limit. In this case, your cache folder will continue to grow endlessly as you visit more worlds and more files are downloaded. Use this option with caution, since with the disk cache disabled the cache folder can eventually grow to several hundred megabytes in size (depending on how many different worlds you visit) and possibly even fill up your entire hard disk!
You can also have Outerworlds automatically delete files in the cache folder that have not been used for a certain amount of time, regardless of the current total size of the cache. This option is disabled by default. If you enable it, you should pick a reasonable value for the number of days old a file has to be before it is deleted. 30 days is a good value. If you set this value too low, files may get deleted too soon and you will wind up re-downloading data unnecessarily and reducing performance.
The total size of your cache folder (determined from the last time Outerworlds scanned the entire cache folder) is displayed at the bottom of this dialog.