Android app packages or APKs are a mix, literally and figuratively. They are usually large because a single APK can contain all the data applicable to all variants of the application, even if some variants or architectures do not need the other parts. At the same time, this type of package makes it trivial to redistribute APKs and transfer them outside the Google Play Store. Things will change next year, when Google starts applying its new Android App Bundles (AAB) format and, while it is generally a win for most end users, it can cause headaches for developers and some users as well.
In short, an Android App Bundle would serve the appropriate APK for a specific architecture, for example, ARM versus x86, and would include only the code, language, layout, and other pieces of content appropriate for that architecture only. This will drastically reduce the size of APKs, which means faster downloads and space saving on local storage. In one case, however, it can be more problematic for some users.
XDA notes that AABs will make it more difficult for developers to redistribute their apps on other platforms or app stores because this APK sharing process works only on the Google Play Store. This will also make sideloading an APK problematic because it may not have all the parts needed for any and all architecture, whereas the previous APK was a one-size-fits-all package.
Application developers may have very little choice, as Google is requiring all new application submissions to use AAB as of August 2021. Existing applications that will send updates will not be required, but will need to target the Android API level 30 (Android 11) or later. Applications that use the Instant Experience feature will also need to switch to Android App Bundles.
AABs come with other benefits, specifically Play Asset Delivery for applications larger than 150 MB, such as large games. These are admittedly convenient features for most users, but they come at the cost of being forever linked to the Google Play Store and services, which is exactly what Google wants anyway.