Alright, a couple things here:
Things that take Game Data Space:
Worlds
Tiny little bits of Official Modules (from FoundryVTT official catalogue)
Premium Modules (that require Patreon and such)
You can see that is taking Game Data by going here: https://forge-vtt.com/setup and clicking “Select Data Files to Delete”, then looking through the 4 tabs that opens.
It’s quite possible that Module inflated Game Data, but depends very much on how you installed it. If you just gave The Bazaar or FoundryVTT (on The Forge) the module.json, yep, it’s in Game Data. If you downloaded it locally, then used Summon Import Wizard to upload, then it would have optimized the assets (images/audio/etc. as appropriate to the module) onto your My Assets. (see here: Importing Worlds/Modules/Systems - W/ Video Tutorial as it is relevant).
Hope that helps.