Android app packages or APKs are a mixed bag, literally and figuratively. They are often large because a single APK may contain all the data applicable for all variants of the app, even if some variants or architectures don’t need the other parts. At the same time, that kind of packaging makes it trivial to redistribute APKs and sideload them outside of Google Play Store. Things will be changing next year once Google starts to enforce its new Android App Bundles (AAB) format and while it’s generally a win for most end users, it could give developers and some users headaches as well.
In a nutshell, an Android App Bundle would serve the appropriate APK for a specific architecture, for example, ARM versus x86, and include only the code, language, layout, and other pieces of content appropriate for that architecture only. It will drastically reduce the size of APKs, which means faster downloads and space savings on local storage. In one case, however, it may actually be more problematic for some users.
XDA notes that AABs will make it harder for developers to redistribute their apps on other platforms or app stores because this process of splitting up APKs only works on Google Play Store. It will also make sideloading an APK problematic because it might not have all the necessary parts needed for any and all architecture whereas the previous APK was a one size fits all package.
App developers might have very little choice since Google is requiring that all new app submissions use AAB starting August 2021. Existing apps that will be pushing out updates won’t be required to but they will need to target Android API level 30 (Android 11) or later. Apps that use the Instant Experience feature will likewise need to switch to Android App Bundles.
AABs do come with other benefits, specifically Play Asset Delivery for apps larger than 150MB in size like large games. These are admittedly convenient features for most users but they do come at the cost of being forever tied to Google Play Store and services, which is exactly what Google wants anyway.