Why the data can grow is explained in the FAQ, here’s the relevant portion :
I just checked your account and you have 119MB used by the pf1 compendium, which is normal, and your world is using 39MB. The actual world data is 6MB, which sounds correct, but you also have a 33MB dd2vtt file in the world, which is uselessly taking space.
Between the 120MB of pf1 and 30MB of pf1-fr module (yeah, I know, pf1 bestiary is ridiculously large), that puts you at 150MB of systems+modules, which means your 60% must mean you had 150MB used by your game data which is an absurdly high amount. Most of the time when the data grows, it will only be for a few MBs (from your 6MB, it would grow to 10MB, maybe 50MB if you do a LOT of quick token movements in a huge scene), and after a few operations, it will compact (shrink back) automatically. It should never have reached 100% in your situation.
If you have a backup of the exported game before you deleted it, maybe you can check what it was that used so much data? If it happens again, from your My Foundry page, you should be able to see how much data is used by each world/system/module using the “Delete content” button, and let me know, maybe I can investigate it further.
I hope that helps!