After editing a photo (making lighter or darker) I exported them to my desktop in order to burn a disk of pictures. I noticed that the original file size was reduced. When I exported the unmodified version the file size stayed the same but lost the

You can lose some detail by using a lossy format, it depends on the constraints you are dealing with.

Regardless of the format, editing, and whatever you have seen on CSI, as an image is blown up past the original resolution it is going to get progressively worse because there isn't enough information to interpolate. Ideally the image should be as large as you can get away with and then scaled down when it is displayed.

How much reduction are you talking about? If you are using a lossy format such as JPEG, the size would be reduced due to more compression (more light/dark areas), if there isn't much difference it could just be trimmed metadata.

What are the pixel dimensions? I am going to guess that even a 6MB image will not support being blown up to 20x30.

You can, of course, export a tiff that will contain as much info as possible, but it will be larger than 6 MB.

Did you click the little triangle next to the "kind" box? That will open up more options and let you select maximum quality and the original pixel size.