The file size is half--a 5MB file becomes about 2.5MB. I even did an experiment of just saving a 'nikon taken' file in Zoombrowser with absolutely nothing done to it. And I got the same "half-ing". And to be more clear: I am only attempting to do basic color/brightness adjustments in Zoombrowser.
Before this, I have only worked with images taken by several different canon cams. This file size decrease has not happened. To further test, I also adjusted a Nikon image in Corel--the file size was only slightly altered as would be expected. My Zoombrowser version is probably 3-4 years old.