Why does my phone on 6.0.1 download the entire app all over again when updating? I have 2 devices, my Cherry mobile One G1 running 6.0.1 (an Android One device) and my old Samsung Galaxy Note 8.0 tablet running Jellybean 4.2.2.

When installing updates on my tablet, only the a small part of the update is downloaded this discussion said they're called "delta". But when on my phone, it downloads the entire app all over again. It's fine for small apps, but some games have well over 100mb of data sometimes and it hurts my data allowance to update them. 

Is it a feature of MM 6.0.1 or is it a bug on my phone?
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.