Has anyone ever figured out why an iOS update size differences [View all]

If I download the update through iTunes it's a couple gigabytes. But if I download it directly on to my iPhone X it's only a couple hundred megabytes.

Anyone know why that is?

Only thing I can think of is that if one goes through iTunes it's downloading the update for every iOS device (phone, pad, pod, etc) and if I update through the phone settings then it's just downloading the file for the specific device. Is that the case or is there some other reason?