How is it not? It doesn't seem reasonable to you to model the expected size of an app as a function with the size of the phone on which the app will be installe

How is it not? It doesn't seem reasonable to you to model the expected size of an app as a function with the size of the phone on which the app will be installed as one of the inputs and the function's first derivative w.r.t. the phone size being always positive? It's not like it's the only factor but of course it would play some role. If you accept a higher size then all sorts of benefits follow. You can make the assets higher res, hold more of their data locally, just take less development time optimizing the app.