Dull Image with increased resolution

Hi All, just wondering if anyone can help with an issue I am having rendering images in Bentley Descartes CE. I find that as soon as I increase the resolution (from 1200 to 3600) the resulting image goes from being nice and 'sunny' to looking like an overcast day.  Same lighting, environment and render settings, the only change is the resolution.  Any suggestions?

This is 1200 wide

...and this one is 3600 wide.

-W