My eyes my eyes.
I have an original iphone in the house, I could not and cannot read on that device. It makes my eyes hurt like the devil. Just does. Its also too small. But then I need my fonts a bit larger.
Until recently I had a MotoG 4.5 inch and it was decent to read on in an emergency. It was still too small. I know have a new phone, a 5.5 inch zenfone 2 laser and its the right size for reading. It also has this awesome zen bluelight filter build into the software and its really great for reading. Well, as great as it can be for me on a tablet type screen. I only use this for epubs I can't figure out another way to read. Otherwise I am all e-ink. I have to read in shorter spurts on tablets as my eyes just start hurting after certain time. E-ink never bothers my eyes.
But I am a voracious reader reading 100's of books a year.