JPEG images opened in Photoshop double in file size upon save. Why?

Question

I've noticed that jpeg files I open in Photoshop (fresh off the sansdisk card) pretty much double in file size (from, say, 3MB to 6MB) after they're saved. This occurs even if I don't change the image size at all and simply save as a jpeg after touching up the levels and sharpness (no major restorative work on the image at all).
Can anyone explain this to me?

Answer 1

JPG format saves disk space by using a coding scheme (called huffman) in which small numbers use fewer bits than small numbers. What is saved is differences between neigbor pixels (the idea being that there is usually much similarity between neighbor pixels). Smaller inter-neigbor differences need fewer bits than large differences. When you adjust tonality, the adjusted image most likely uses a bigger portion of the tonal range than the original image did, and the inter-neighbor differences are increased. A sharpness boost also increases differences between neighbor pixels, particularly so if you boost sharpness so much that noise is increased, and more bits are needed to store the differences.
Another thing is that when you open a JPG image into an image editor, the editor uncompresses the image. When you re-save it, you have to select a compression ratio, and it may not necessarily be the same ratio that the original was compressed with.
Best regards,
Antero