As per title. A simple change in brightness and contrast changes the image size and from what i can tell the quality of the photo taken.
Is this normal or a bug.
To see fro yourself, take a pic, and edit its brightness and contrast from the gallery app. Compare the two.
JPEG is a lossy Picture-Format. Everytime you save it again, you loose Picture-Information. Compression-Artefacts will be more seen and Image will be more blurry and it will shrink in Filesize.
Also the Size of a JPEG-Picture is depent from the content, try it out. Big Areas with same Color result in smaller Filesize, so also often dark Pictures. Its all up to the Packing-Algorithm and the choosen Compression-Rate (100% Quality = No Compression = No Content-Loss = Big File, 0% = Color-Swamp = Much Content ist lost = Very small File 
)
So maybe its better to save a edited JPEG-Picture in a non Loss-Format like PNG…etc or set the JPEG-Compression-Rate between 100-90%.
I don’t think there are any settings like that for the user. Or am i missing them somewhere?
Anyway. Photography in SFOS leaves a lot to be desired.
I generally use Imageworks, mea culpa, and save images in .png. You can do unsharp masking and even color curves. That’s a lot less lossy. But it isn’t native to gallery, that’s true.
EDIT: I just did a three way comparison with a 500 kb image (shot with showmotion). The difference in quality for the same simple brightness and contrast adjustment was not so noticeable. I just zoomed in on ‘cat hairs’ and the artifacts were comparable. The size, png 2.5 mb, certainly was.