I suspect it's the smallest they can do while still meeting the technical requirements. There is definitely a size below which usage would suffer too much for

I suspect it's the smallest they can do while still meeting the technical requirements. There is definitely a size below which usage would suffer too much for many people, but such a device doesn't need a very large UI, since most of its utility is in displaying small bits of information quickly, and extensive data input is performed better on a phone anyway.