The predominant marketing and business philosophy is to build products that can be sold to the maximum number of people. (That is - undifferentiated, one size fits all products over solving specific needs). And since phones have become primary media and computer devices for most people, who want to use it all day, it makes more sense to make bigger phones with all the features and big batteries. Companies will say consumers want this kind of device because this is the only kind of device they make which is also usable (same thing with SUVs and crossovers in auto industry), but ignore other large consumer groups who want small phones, custom ROMs, interesting designs, etc etc.

Well relatively small compared to their normal model which is unusable single handed. (Buyer remorse kicking in already)

Apparently oneplus is planning to launch a mini version of their latest phone.