Why do my graphics appear pixelated on ios?

Hi, I'm new to mobile development. I recently published 2 games to android and to ios. I tested them on the galaxy s4 and the note 4, the quality looks amazing. Once I tested them on the iPhone 5, 6, and 6+ the graphics looked pixelated. The quality was nowhere near the same. In the publish settings, I set the resolution to high.That improved it a little but the quality still isn't as clear as android. I've searched high and low on google and found nothing useful. I'm completely stomped.Can someone please help me with this issue?