Tech team ran a file size analysis on III a while back, which indicated that there's no pages that are really taking up a lot of space particularly, and the growth of total storage against time is pretty linear. Individual file size limits and upload limits per page ensure that there is no single culprit taking up a lot of space, or even a single user.
The effort required to make a sandbox IV is far less than the effort required to select, notify creators of and remove files from II and III. I'd also wager that increasing the storage limit for I might not be worth the confusion caused by deviating from the current pattern of sequentially increasing sandbox numbers
Edit: we used an external script that uses the wikidot API to check the files of each page. There's no wikidot-native tool that can do that.