Why does Preview display an edited image size smaller than the actual size in Finder?

I have recently purchased a new 15" MBP and have noticed that when I export an image from Photo to my desktop and then reduce its size in Preview, the file size displayed in Preview is much smaller than the 'actual' size displayed in Finder. With my old MacBook + iPhoto combo, they were the same. How can I get the actual size to be the same as the Preview size with my new MBP + Photo combo?