A client sent me a 420 KB photo, saying the file size was too low for a web form with a minimum requirement of 1 MB.
I opened it in Preview app, chose File > Export… and the JPEG quality slider was initially at roughly 80%. At that setting, the new file size was higher at 503 KB. If I dragged the slider to Best quality the file size increased to 1.5 MB.
Unless I'm incorrect in thinking this, the app can't add any new image data to increase the quality of the JPEG, right? I understand it might add some for a thumbnail but the photo already displays one of those. So what's Preview adding to the file that would increase its file size at all, let alone by over 300% for Best quality?