Hi
I’ve run into exactly the same situation a few weeks ago. The short answer is that you need to disable “Cache all images” as its not possible to just cache images without them going through the optimisation process first - which sadly bulks them up.
I’ve written a bit more detail on an similar post here: https://discourse.getgrav.org/t/newbie-question-from-germany-images-in-cache-twice-as-big-as-originals-feature-or-bug/6045/5