According to a new report out of South Korea, Samsung may be looking to go in the opposite direction with a Galaxy S9 Mini. This would be a premium device in the same mold as the expected Galaxy S9 and Galaxy S9+, but with a 4-inch-class display. The Galaxy S9 Mini would retain the design hallmarks of the Galaxy family including the curved display edges and minimal bezels.
We should take this news with a huge grain of salt, however. The rumor-mill also promised that there would be "Mini" versions of the Galaxy S7 and Galaxy S8, but those reports never materialized.
Premium smartphones with displays smaller than 5-inches have largely been abandoned by manufactures due to lack of customer interest. However, Sony does product the Xperia XZ1 Compact, which offers a potent Qualcomm Snapdragon 835 processor in a 4.6-inch form-factor. In addition, Apple still produces the iPhone SE which features a 4-inch display and iPhone 6s-class hardware.
Speaking of the iPhone SE, the smartphone is likely up for a hardware refresh next Spring, so it will be interesting to see if Apple resists the urge to make the screen larger and sticks with the tried-and-true 4-inch form-factor.