It's more of a problem on Cluster Cam (and Constellation Cam) than Galaxy Cam. Particularly with the brighter star clusters like the ones in this image.
Although Cluster Cam has a smaller aperture than the main scope, its field of view is wider than its aperture is smaller. This acts to focus about 4 times more light onto each pixel for the same exposure time. Our CCD's are very sensitive so this can easily cause blooming on the brighter stars.