Image size changes during playback?

I have a Vivitek H1080FD 1920 x 1080 DLP projector with a 100 inch 16:9 screen. I have a blu-ray player hooked up along with an HD Satellite DVR all routed through a Sony 7.1 receiver. During playback, the image size will sometimes change by about 3-5 inches usually in height. This happens during playback from all sources. Is there some setting on my projector that would fix this? Everything runs through one hdmi cable to the projector. Any help would be greatly appreciated.

Thanks.

This problem happens with all media, tv, dvd, video games, etc. The size will change throughout the same program from the same source which is what I thought was odd.