I'm using Android Studio 3.0. When I Analyze my APK, I see the following
Raw File Size: 4.1 MB, Download Size 3.3 MB.
Why is that different?
Since this is an Instant App, it prevents it from being uploaded. How could I shrink the Raw File Size without need to change my code/resource that is needed on download?
The APK Analyzer provides two sizes for each file which is packed into your APK, as well as overall for each 'directory', including total. These sizes do not indicate the compression that you got when the APK was created (APK format is ZIP compatible, so you can use unzip -lv qqq.apk to examine what effect this step had for squeezing the total).
For the end users, both sizes actually matter. With smaller Download Size, they get your app faster, and waste less off their data plan. With smaller Raw Size, the APK takes less space on the user's device.
But there are some tiny details to take into account. Let's take for example the native libraries (if your app uses NDK).
First of all, consider application update. Google Chrome team found that if they don't compress the native libraries, app updates become quite a bit smaller. If you multiply this gain by the number of expected app updates, this gives a major advantage.
Second, if you don't compress the native libraries, you can actually reduce the disk consumption of your app (for Android Marshmallow and above), because then, the libraries can be loaded in-place, without being unpacked to /data/data/your.app.package/lib.
Luckily, all this is managed by simply adding an attribute:
android:extractNativeLibs="false"
to the application tag in your AndroidManifest.xml file.
Same considerations may be applied to the assets: AssetManager can load them from inside the APK, without need to unpack them to disk - if the asset is not compressed in the APK.
APK Analyzer will show you how Google Play will squeeze such assets for download.