I've noticed that if the content width variable is not declared within a theme's functions, Photon will try to request an appropriately resized image. How it does this, I have no idea. Photon really sucked at its attempt in my case, and it requested 540px images for a 716px wide content area. The resized images were very blurry once stretched by the stylesheet to 100% width.
I fixed this issue by declaring a content width, as described here.
Photon now requests images of the exact width of my content area and links to the full-size versions.