As an app developer (both Android and iOS), I can tell where. When I’m done with the basic app features the app is no more than 3-5 MB but then I can’t ship it without some libraries that make me money - analytics and ads. Then again, if I only integrate one or two ad network SDKs, the revenue from the app can’t even support its development. So I add 12-15 different SDKs from all the ad networks and have them compete for the ad space. Then again, I need things that track crashes, and then offer in-app support, then someone emails you that you can silently earn $xxx per day only by integrating their SDK. Why not? Who doesn’t like an extra $xxx a day?

See what I did there? I turned that 3MB app to a 80MB app.

Disclaimer: Most of my apps are ~15MB and I try to put them under 20MB on Android.