I had edited an original jpg file 5.61mb without cropping, just change the brightness.
when I saved the image, it became 1.58mb.
Image was saved as 8 high quality jpg.
why such a big loss in size?
compression as in discard data, higer the compression more data is discarded.
and if your image has very simple background, or not having lot of details, physical file size will be smaller, eg, a glass of water against plain color background vs night cityscape of Orchard Road.
you should start shooting raw than jpg. Raw file give you more details for post processing. Upon converting to the highest quality jpg the file size will be reduced by 50% or so. File size is relative to the info (color, bit depth, saturation), dimension and resolution of the image.