How do I make color change in the iOS's native image editor without losing image size/quality?

I create non-professional/hobbyist digital art. Recently, an author approached me to create a book cover because he likes my digital paintings. This is my first book cover. The image requirement is 3000x3000 pixels. I finished the work and the Photoshop file (PSD) is 3000x3000 pixels. The artwork is fine, but I wasn't quite satisfied with the overall color/tone from my tweaks in Photoshop. If I open the image on the iPad and go to **Edit**, then use the magic wand, everything looks beautiful. It's exactly what I want. However, any edits I save on the iPad (then email myself) are much smaller. According to the info on my Mac, the original JPG is a file of 3,573,894 bytes (3.6 MB on disk). After color change/save in iPad the image JPG is 1,538,390 bytes (1.5 MB on disk). Additionally, when you zoom in, the iPad-corrected file is noticeably grainy compared to the original. This auto-color correction on the iPad looks so good. Is there no way to preserve the original size/quality when using this feature?