Why is Apple retaining the small screen size when Android users get the luxury of indulging a big screen?
If Apple really is just extending the length, then it's legacy app issues. They're afraid of breaking hundreds of thousands of apps.
When people talk about "polished apps", they usually refer to them being designed specifically for just one screen size. This was seen as a huge advantage over other systems supporting multiple sizes. And it is, until you want a different size screen or denser PPI. Then you end up with non-expandable methods like pixel doubling.
Basically, Apple's gotten themselves stuck in much the same situation that some other phone manufacturers were in for years. They're going to have to start a huge push towards resolution independent apps if they want to break free.