There is demand for larger phones, yes, but manufacturers also charge more for bigger devices and most of that is margin. Following their own logic, they also charge less for smaller phones.
If your customers are sticky, then many of the people who buy the smaller phone would have otherwise bought a bigger phone for more money. Introducing a smaller phone brings down profits.
Why do they do that though? Usually more compact, high end devices would be more expensive than bulkier one. When has this trend reversed?