Slight file size discrepencies between identical photos?

How did you duplicate the images? A straight copy should definitly give identical sizes.

You're saying these are separate images taken in the camera? If so what you are seeing is normal. No two images shot in the camera (unless under strict lighting and composition control as in a studio) will have the exact same kbs size.

The image dimensions will be the same and the images will be the same size if you display then at the same settings but the kbs will be different.

This holds for RAW and JPG but the difference will probably be greater with JPG.

These are the exact same images. I just imported the same image multiple times.

In that case then I can't help you.

If I import the same file multiple times all data in Aperture are identical. Filesize, MP size, image dimensions. There would be no reason for any other result.

How are you looking at the data, in Aperture?

These photos are several years old.

Have they been copied between drives with different filesystems? Opened and saved again?

Copying files between drives even with different file systems should not cause the files to change size. If that was the case then every time you copied a file you would be changing it.

You might see different sizes if you looked at the file in one place and then in another but two files with same size on file system A should show the same size relative to each other on file system B

They have been copied between drives with different filesystems (iphoto and aperture and old macbook to new macbook). Maybe this is the issue?

How did you transit your images from iPhoto? Did you export them from iPhoto or import the iPhoto library? Have the images you are comparing been imported from iPhoto at the same time, from the same iPhoto versions?

If you compare the original master image files of the images in question in the Finder, do the originals have the same size?

You might see different sizes if you looked at the file in one place and then in another but two files with same size on file system A should show the same size relative to each other on file system B

Yes, but are the files on the same filesystem now? Or referenced on different filesystems?

Opening and saving JPGs can cause the file size to alter. Every time a JPG is saved, a new 'lossy' compression is performed. Thus openening and saving JPGs numerous times will besides from degrading IQ also shrink the file in size.