Apps' functionality is often simple, but the apps themselves aren't. Back in the ancient days, iOS used to refuse to download apps above a certain size via cellular connection. It was 10 or 25 MB. I don't think I have a single third party app on my phone that's less than half that size.
These well-crafted iOS apps still exist. E.g. Overcast is 7.3MiB. But yeah, most apps use a lot of frameworks and assets and they can be hundreds of megabytes.
