I wouldn’t worry too much about the file size when converting from .dng to Jpeg, as you’re probably already aware, jpeg’s use an algorithm and compresses the image - this happens every time you save a jpeg, so if you save a jpeg, then save that jpeg again it will compress it even more but a file size doesn’t mean the image quality is any less, it’s just the algorithm doing what it thinks is best. At any rate, if it’s for website viewing you tend to want the file size to be as low as possible for faster loading times. Take a look at Amazons home page, the images look high quality but are usually between 30kb-100kb To answer your question though, I must be honest I don’t know why it’s spitting out two different sizes unless you’re saving the jpeg, then re-saving the jpeg.
... View more