Responsive Image Module not reducing the file size when resizing the browser (Drupal 8)
I guess the 685kB is your original, uploaded file and the 1MB file is generated on the webserver by GD or ImageMagick.

When your original file is heavily compressed and at the same time your JPG quality setting is very high, the generated (pixel-smaller) files can be (byte-wise) larger than your original. There is not much you can do against this except reducing JPG quality settings in Configuration &gt; Media &gt; Image Toolkit

Another option would be using a modified JPG library like MozJpeg, but this is beyond the scope of Drupal.