Iphoto '08 NEF file size info missing

Some of the photos that I have taken on my Nikon D50 (which are imported into iphoto as NEF files) show "0 KB" as their file size. So I have no way to figure out how large an album is. Iphoto told me that one album has 356 MB of pictures and when I went to burn them on a CD in Toast 8 the size came up as 5.6 GB (which sounds accurate for 200 pictures at around 5 MB each)!
And some do show file size; I thought it might be related to whether or not I had made adjustments but that doesn't seem to be true. It just seems random.