Why is iOS install size so much larger than Android?

Referring to the installed size of the app, not the download size.

We have an app that is roughly 100 MB installed on Android, yet over 320 MB installed on iOS.

What can we do about this?

We have already tried the steps recommended by Unity.

We do have a significant amount of 2D sprite sheets and background textures. However, all of these are set to ETC2 compression for all platforms. The way we understand, even though not all devices support ETC2 natively, the textures are still stored in the compressed format, and only decompressed in memory. Please correct us if that is wrong.

Also the Unity editor build log reports numbers that are no where near the 320 MB mark.

Textures      139.5 mb	 80.9% 
Meshes        0.0 kb	 0.0% 
Animations    0.0 kb	 0.0% 
Sounds        0.0 kb	 0.0% 
Shaders       227.0 kb	 0.1% 
Other Assets  20.7 mb	 12.0% 
Levels        4.3 mb	 2.5% 
Scripts       1.4 mb	 0.8% 
Included DLLs 5.1 mb	 3.0% 
File headers  1.1 mb	 0.7% 
Complete size 172.5 mb	 100.0% 

Where is the extra 220 MB coming from?

You must be building universal architecture app. And it doubles almost everything for 32 and 64 bit devices. See here