The way this works on Windows (I think) is that internally, "cut" means "copy file path to clipboard", and "paste" means "move file from old place to new place". Thus, if you want to cut and paste from "Downloads" to "Documents", then it doesn't matter how big the file is: you just change a couple of directory entries without ever changing the file. If you want to do this across file systems, it has to do a copy.