Here is the Google documentation on AABs:
Google Play’s new app serving model, called Dynamic Delivery , then uses your app bundle to generate and serve optimized APKs for each user’s device configuration, so they download only the code and resources they need to run your app. You no longer have to build, sign, and manage multiple APKs to support different devices, and users get smaller, more optimized downloads.
So as far as I understand it, the AAB might be larger, but when the phone installs the app, Google Play will generate the APK for that specific device, which will be smaller.