But, it depends on how much motion there is. If you record a video of your desktop and you're just editing a word document or looking at a picture, or moving your mouse around, etc., then the video won't take a lot of space (basically not a lot of movement). However, if you're recording a game, there's constant movement, and only certain times that its still. So you can't really compare 2 different videos that are of different things.
