There is different terminology for it that can explain it in more detail, but if you play your game exported as NWjs, you will see that it has 3-4 processes in your task manager. One of them is the memory use of what's currently active, shouldn't take too much memory (what the plugin can affect). Then there's a big one, which can easily be up to 1-3 gb depending on how big your game is. This one holds every sprite, tile and preloaded sound file. Notice that nothing you do reduces the size of this beast in any remarkable capacity. It's loaded in on startup.
The only work-around is storing images in the files folder and loading them into a sprite or tile object during runtime. However once it's loaded in, it can't be unloaded. So you can only postpone the memory bloat.