Video clips posted on the Internet are created by many different individuals with various types of recording devices - camcorders, camera phones, digital still cameras with MPEG recording, etc. Depending on the recording device and its specifications used to record video clips, the quality of the video may differ from one video compared to another.
When using the Full Screen function of the TV, the size of a video clip is expanded to better fill the screen. However, expanding the size of a video makes it easier to see imperfections (like pixels) in the video footage and reduces its image quality. Although this is normal for any video, it will be more noticeable in lower-quality videos.