canon lv7490 dimming issue

Hi all,
I've just recently started using the canon lv7490 that I purchased almost 2 months ago while our space was being built. I use Proselect on Mac and am connected to the projector via a mini display port to via connector. The projector is set to Cinema mode and at full screen the images look great. As I cycle through the sizes and images the projector seems to be changing the brightness of the images. For instance, when I switch to actual size mode in preselect the larger the image the brighter it seems. i know this can be a little bit of an optical illusion since there is more image on the screen so I've isolated parts, an image with white at full size seems to lose contrast and have not as bright of white when it's reduced in size. this is a phenomena that I don't see in any form at my other location with my panasonic projector, same computer and same software. Canon's not at all helpful suggestion is use another computer.
any ideas? both projectors are mounted permanently in each location and it is not feasible to cart them back and forth.
thanks
MArk