The Pocket Operators are just a generic DAC+amp chip, the Cirrus CS42L52. The DAC is driven by a 48MHz MCU with just 32K of RAM. If it were purely a question of technical / hardware capabilities it would all fit into an app ten times over.

http://hackingthepo.weebly.com/ has a terrifically detailed teardown of a PO-16. Or SiLabs (the MCU's manufacturer) did a marketing video / interview with one of the Pocket Operator's designers about the hardware design: https://www.youtube.com/watch?v=ZjrOnlMkJHU