But they don't necessarily have to be. We've never had an option where, for the same price, you could choose screen sizes. There's a sizable portion of people for whom larger phones are simply difficult to use. Would I have paid an extra $50 for my iPhone 12 mini? Yep, because I was buying based on the size, not the price.
This is so weird compared to 20 years ago, where the smaller the phone was, the more expensive it was. The big bulky phones were a sign that you couldn't afford the smaller one. A few friends joke that I couldn't afford a 'real' phone when I pull out my 12 mini, which... is nuts because I bought it outright, and a couple of them worry about 'when can I upgrade? oh, let me check how many more payments I have on this current model'.