That sort of behavior (a world exploding in size, average being about 10-20 MB per world) is caused by a module.
What you can do is to download the world, then look through the db files to see which one(s) have exploded in size. Edit those db files to remove the bad data, and save them. At that point, test locally in FoundryVTT and make sure everything works. You can then delete your world on Forge-vtt and re-upload the world. Also make sure your modules are disabled so you don’t hit this issue again, and try to determine which module caused the issue.