As UHDTV programs are increasingly available, there are increasing needs for UHDTV signal measurement tools, which can estimate the perceptual quality of UHD signals compared to lower resolutions (e.g., 1080p). In this paper, the perceptual video quality of UHD signals is compared with other formats (1080p, 720p, 540p, 360p) when they are displayed on a UHD display. We performed a number of subjective tests using a UHDTV display at various viewing distance and the subjective scores of the various picture sizes are analyzed. Then, we propose some features that can be used to estimate the perceptual quality improvement of UHD signals compared to the lower resolutions.