Image size is canvas size. X pixels wide Y Pixels high. Files sizes vary all over the placed it depends on what is recorded for the image so file size will vary with file type and how well data compresses. So your 12mb Image file that has a 70mb canvas size. Each pixel requires 3 Bytes in 8 bit color or 6 Bytes and 16 bit RGB color. Your canvas size is 3x*3y=70mb or 6x*6y=70mb you image also has an aspect ratio you need to know so you can solve the equation for x and y.