How does saving a file as alow (vs high) format JPEG reduce the file size? I saved both as a test, opened in photoshop and looked at image size: the resolution and pixel dimensions are the same while the file size differs.
Changing jpeg compression from high to low never affects the number of actual pixels in the image, but it does affect how highly compressed the file is when saved and how many visible artifacts you might see. The change is going to be in the size of the file saved to disk. Check out those numbers and that's where you'll see the difference.
And it effect quality of final picture,....higher compression, lower quality,…jpg artifact and details breaking down.
Best is edit image in Tiff, or PSD, and final edited version you can save on jpg, lesser compression better.