I am well aware of the sizes of the stores. Are you aware that the relative amount of malware is is not merely proportionally less? Are you aware that F-Droid-style reproducible builds are not possible on iOS? People using an iOS device can never be sure they are installing the secure app they wanted to install or some switcheroo.

On the other hand, Google's and Amazon's app stores are not open source but built for an open source platform, together have far more users than the Apple App Store and far fewer malware installations. Discovery in the Epic case dug up some documents showing that Apple had no dynamic analysis for App Store apps at all. Google and Amazon both run Android VMs in their datacenters to catch fishy behavior that can't be found via static analysis.