People buy the iPhone for iOS and the iPhone build quality.
We are limited to a 4" screen because that is the only thing that Apple offers.
To make this comparison to Android phones based on screen size is silly. What it says is that, given the option, most people prefer a bigger screen.
What this tells us is that Apple is behind the curve. There are a few supply-chain-based rumors on here already claiming a big-screen iPhone is in the works for next...