BTW, I forgot to explain why Android specifically. On that OS, the apps can override the function of the volume key, and most readers - including Kindle - use it to flip pages. So you can hold the phone with your thumb on volume down, and read with minimal movement after finding a comfortable position. Ideally, this also needs a phone that is small enough to be held comfortably in one hand.

There's no similar feature on iOS, so you need to flick the screen to flip pages.