Why does my phone on 6.0.1 download the entire app all over again when updating?
Google Play Store's App Updates vary by device's Android version. It's highly likely that the new updates being installed on your Android 6 device has huge changes in it to support latest Android APIs.

If you're downloading big games, there will be some extra size
 to be downloaded initially.
But if you're updating the app, it usually requires much less data.