The Blue-Berries are getting smaller as Opportunity climbs upward. And since these are sedimentary rocks, that means the larger Blue-Berries are older. They grew larger in the past?

I can see a chemical explanation and a biological one.

For the chemical explanation- basically, as time went on, less of the chemical for making the blueberries was around.

For a biological explanation- basically, as time went on, the environment got more hostile for whatever life was making the Blue-Berries, so they grew smaller and smaller until they dies out. This would conform with the area being wetter, the air being thicker, then gradually the water dried up and the air thinned.