a question from the Twilight Zone of digital photography

Here's an interesting occurance. A photographer shot a wedding for me using a Canon 20D and a Digital Rebel. Both were set to "fine, maximum" (I believe that's the description he gave me) to give us the largest JPEG size and quality. We have images shot in sequence on both cameras. And we have the same "problem". One image is 434 KB the next is 2.7 MB. No difference in the subject or light or environment. How did the camera shoot one image so small and the next so big? When photographers who shoot for me using Canon gear (I use Nikons) shoot with the same settings we get images with 2-3 MB file sizes. What happened?