If I had to bet, it's probably Apple finally getting around to offering a phone product in the size that people want. For years, if you wanted a large phone your choice was only Android.
And now the situation is reversed, if you want a normal size (ie, less than 6 inches) phone, Apple is starting to be the only company in business that delivers powerful hardware in a non-phablet form-factor
I assume you're comparing only Nexus devices, because the breadth of Android devices in different sizes is by no means limited.
Yeah, I've got a Moto X and while it's not particularly small, it's basically the non-phablet version of the Nexus 6 with a few minor differences.
The trouble being that small devices are practically always very slow and forgotten the day they hit the market.