exposure varies with resolution of render (particles)


I have been working through this [ http://www.blenderguru.com/the-secret-to-creating-realistic-grass ] tutorial posted at Vimeo and embedded at Blender Guru complete with the source files by Andrew Price.

I am relatively new to Blender and this is my first introduction to particles. Rendering the finished tutorial I have noticed that the effective exposure (by which I mean the shape and order of the histogram) of the grass seems to be dependent on the pixel resolution of the render. The larger the render the darker the grass.

Am I likely to be right in thinking that this variation in the exposure relates to the use of particles ? And / or are there any of possible explanations for the image getting darker, the larger the resolution.

Is there some keyword/s I should be Googling for an explanation of why I am seeing this or how to go about resolving or working with the issue ?