What's with all the "unnecessary" data in Android 6? So I just finished updating my phone from Android 6 to 7.

Before downloading the update, I had two point something GB of free internal storage space.

The update was one point something GB of size, and I expected to get about as much back after the update was concluded.

Except, I have now - even after updating all my apps - five point something GB of internal storage available ... and no, I don't think I'm missing any apps (nor personal files, since I'm in the habit of storing as much as possible [read: everything] on my external SD card).

I'm certainly not complaining, but I'm curious. What's the (most likely) data that's been deleted? Is it really possible that Android 7 is that much more efficient?
Runtime environment is different... Marshmallow pre-compiled applications and stored them for use, Nougat compiles on the fly as needed using a JIT (Just In Time) method. Seeing a gain of a few GB is not unusual after upgrading to Nougat.