There is no marketing. Bigger is just better. I got the regular sized iPhone and every time I pick up the pro max it feels good. It’s visually more striking, it’s nicer to use. It’s just a little harder to hold but with two hands is still fine.
I mean it's not like this has been a sudden shift, phones have been getting gradually larger over the years. I seriously doubt manufacturers would be using more materials to make a smaller amount of larger phones instead of more smaller ones if the larger versions didn't sell that much more.
It is a business, though, and if 5% of sales cost disproportionately more than 5% to produce, market, support, and all those things, then it’s fair to say “it doesn’t sell” and axe it.