This is a difficult subject. When converting an image in JPEG, there are usually 256 parameters and an important decision, which influance the size and quality of the output. The "quality" settings (a number from 1 to 10 with DPP, or 1 to 12 with Photoshop, or a percentage, or whatever) are totally arbitrary and not comparable between programs; they are only for the user interface.
Increasing the "quality" means increasing the file size compared to a lower "quality" setting of the same program. However, an image with higher file size created with one program is not necessarily better, than a smaller one created by another program. Even worse: the resulting quality depends on the image content as well; not all images require the same JPEG encoding level.
Perhaps the best is to create JPEGs from images with very fine details, straight lines, plain unicolored, untextured patches with different "quality" settings. Then open the JPEG images in Photoshop or whatever and look at the critical parts in 300%. (Note: saving and reopening the images is absulutely required, otherwise you see the original content without the JPEG degradation.)