Scott Hurff explains that “Apple’s iPhone 6…officially signaled the Dawn of the Era of Huge Screens.”
The “natural” zone for your thumb has changed; it’s harder to tap things at the top of the screen, for example. If you upgraded to an iPhone 6, you probably noticed this in the first few minutes.
Be sure to design accordingly. Don’t put important buttons / icons / targets in areas that are hard to reach.
P.S. You may not be aware of an iOS feature called “Reachability” – double tap (don’t press but just tap) the Home button and whatever app you’re using will slide down within easy reach of your thumb. See here.