Hi there 
The world data wouldn’t show in your Assets Library, since that would usually contain only the assets (images and such), but you can find it in your world once you’ve exported it from the Game Configuration page.
This article goes into some detail about how to go about that World Size Check
The best way to check the individual world’s data is to do an export, and then check inside of the exported world/data directory.
If you’ve cleared the world’s chat log in-game, it should be the same end result as exporting, deleting world/data/messages, and re-importing, unless something is preventing the Foundry operation from clearing the chat log perhaps. Did you re-export after clearing the chat log and extract that to confirm?