How big is the JSON file? Try saving it to your desktop using the "Browser Download string" action. Then, analyze it with an online tool to see which objects are taking up so much space.
My guess? The dictionary.
You’re writing saveJSON into the dictionary. Then, 15 seconds later, you create a new save, which stores the entire dictionary (already containing the previous save) into saveJSON again. After that, you write saveJSON back into the dictionary.
It’s like Russian dolls — your dictionary keeps nesting copies of all previous saves.
The solution is to clear the dictionary before running "System save".