That’s the problem with monopolies. They’re the company offering iOS devices. So if they suddenly make only big phones then there won’t be any demand for small iOS devices because none are for sale (new old stock doesn’t count). It automatically makes the big phone choice a winner simply by incumbency. As a result, Apple cannot know the size of the small iOS device market.
The 6s and SE were their last remaining headphone jack phones. Maybe they’re trying to force the wireless headphones change as well as pave a way for new models. The SE actually sold more this year. But adding it’s 8% made it a clear 33% model replacement.