JPEG is not a faithful round-trip format. That means if you read a JPEG into memory, and then write it back to disk, you won't get exactly the same file. The file size can differ, and so can the colors being used. The latter point especially is important to remember. If you need faithful round-tripping you need to use a format like TIFF or PNG.
As an aside, given today's disk sizes the difference in size seems inconsequential.