YouTube for iOS now supports 1440p resolution on devices with screens size of 4.7 inches and 1080p on screens measuring 4 inches
For a long time the resolution of YouTube videos played on iOS devices equipped with displays measuring 4 or 4.7 inch, it is limited to 720p. It was understandable because it was consistent with the resolution of the mentioned screens. Recently however the situation has changed: now YouTube for iOS on devices with screens size of 4.7 inches (iPhone 7, iPhone 6s and iPhone 6) supports resolutions up to 1440p, and devices with screens of size 4 inches (iPhone 5s and iPhone SE) — up to 1080p.
Of course, the screens did not fit more pixels, but the increase in resolution has improved the picture quality. Note that it is achieved by increasing the amount of data transmitted wirelessly, so in cases when you have to pay for the bandwidth, the user can, as before, to prefer the lower resolution.