Generally speaking, if you have a 2560x1600 or 2560x1440 monitor and you watch movies/videos on it you can do it in two modes; full screen and windowed mode.
Becasue 2560x1600 and 2560x1440 resolution is above Full HD 1080p (1920x1080), playing a video in windowed mode will naturally not fill the entire screen. When you play it back in full screen mode you are basically stretching the movie/video to fit the monitor's resolution. The more you stretch it the worse it will look.
Stretching a movie to larger than it's normal resolution results in something called video interpolation which basically estimate how the entire movie should look. It's actually a bit more technical than that. Therefore, stretching a HD movie onto a 2560x1440 monitor will result picture quality that is a bit fuzzy. That's because 2560x1440 has almost 80% more pixels than 1920x1080.
DVD movies (1280x720) will look worse when you stretch it to fit a 2560x1440 monitor. That because it will be stretched to 4x the original size. Again that's due to video interpolation.
As an analogy, let's say a hand knit wool blanket represents the video. When playing back the video in full screen mode you are basically stretching the video. When you stretch out a knit wool blanket, the wool yarn starts to separated from each other and small holes begin to appear. The more you stretch the blanket, the bigger the holes will be until you can see through the holes. That's basically what video interpolation is like.