Premium Account download size exceeds file size

why do some downloads exceed the file size and keep downloading past that number. a file size can be 1.2 gb but the app keeps downloading past that and can reach up to 15-20% more. sometimes it eventually stops and downloads completely, and other times it doesn’t stop downloading and eventually times out saying “download link expired”. this happens sporadically with various websites and files. it’s a general frustrating failure that happens, not to any specific websites. it’s in general