After using Ewww for the first time I had trouble understanding why my Ewww image file size wasn't comparable to my "Save for web" Photoshop image file size. I'd start out with the same huge stock photo, run 1 copy through Photoshop, run 1 copy through Ewww, and then compare files sizes. In Photoshop I was scaling my image down proportionally to 1200px wide. I realized I also needed to do this with the huge images I uploaded to my media library. I had to go into the media library to edit image & scale it down to proportionally to 1200px wide (the max my theme allows). BINGO! When I compared the Ewww results they were very very close to my Photoshop "Save for web" results. Now, I have an easy solution for my clients to be able to optimize there own images without having to teach them too much! Excellent plugin!
Update: Still loving this plugin! Now it's become a way for my clients to upload photos & not have to optimize them beforehand. It could not get any easier. I really really appreciate all the quality of this plugin!