Shouldn't be ... perhaps just a perceptual thing ? If you are looking at the 55" C3 though you may want to consider the more expensive G3 model which has higher peak brightness. Unfortunately LG don't make that screen in smaller sizes.Is the 42 inch noticeably less bright in HDR compared to 55 inch? Or is the difference merely because of the bigger screen, meaning the bigger the screen the more impressive the hdr is?