Set the quality to 70% instead of 100%: q: 70
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I didn’t test the ImageOptimizer addon, yet. Maybe that helps.
Besides that, I’m aware of that bug since a few months and I didn’t find a real solution, yet. Setting the qualitiy helps for jpg files, but not really for png files. So, when I have full control, I replace some png icons with svg icons and I upload png images in the needed output format. When I don’t resize them, the original file is delivered. It’s a bad workaround, but the best one, I found for png files so far.