My specific vision for iOS Sixis was as a digital tabletop game, where the device would lie flat among the players gathered around it. In my experience, this just doesn't carry well to the small screens of phones, so I decided early on to focus just on iPad (at least for this edition's initial release).
And, yes, an iPhone edition would require a different UI, one optimized for use by a single player holding and viewing the device at a time. That's fine for single-player mode or network play, but this isn't what I was aiming at with v1.0.
I do appreciate the feedback! It helps a lot in guiding what decisions I'll make for where to take this app next.