Welcome to digideep
You may have set it to save as a smaller JPEG (or with higher compression than what was shot, there are different levels of JPG compression) so even though the size and the rest have not changed, the compression scheme has. You should check to see what options you can use for saving and setting the compression.
You could also save to a non-lossy Codec such as .tif and use that version (after adjustments are made) as a Master to work from to make copies for the Web, Email, etc.
Which Paint are you using?