Can you stack them so that their edges are all aligned on one side so that we can see how much longer they are, relatively speaking?
I've seen photos like this, including on Engadget's recent review of the 17" MBP, but the edges don't appear perfectly aligned. Perhaps it's just parallax error on my part. I'm not 100% sure though.
If it's too much trouble, don't bother. Your photo is already pretty good. 🙂 For some reason, I always think the 15" isn't much bigger than the 13", and the 17" isn't much bigger than the 15". Therefore, the 17" MBP isn't big at all. 😛 Well, it's still 17" diagonally, so I must be wrong.