Apple uses a bit more bezel on their phones, so in physical dimensions they are closer to some of their Android competition than you might think based on screen size.
[1]: http://www.phonearena.com/phones/size/Motorola-Moto-X,Apple-...
Nexus 5: 137.9 x 69.2 x 8.6 mm (5.43 x 2.72 x 0.34 in)
So 63 cm^3 vs 82 cm^3 in volume, and 92.5 cm^2 vs 95.4 cm^2 in front area.
In this context, the iPhone 6 is actually smaller than the Nexus 5, by about 20%.
I've held both and they feel identical. Same vertical and horizontal dimensions and same weight. The thickness isn't even noticeable in your hand.
Using screen-size as a measure of physical size let Apple pull off a pretty epic PR coup. The iPhone 6 is essentially the same size as modern 5" Android phones, but has a significantly smaller screen.
I'm sure Apple does market research, but they've proven to be very susceptible to their own dogma.
I have a 4S still around as my "iPod touch" and it does fine compared to my 5 in terms of usability for apps and music. Remembering back, I didn't somehow feel when I got the iPhone5 and later my Nexus5 that my 4S was "inadequate". I did feel that it weighed a lot compared the iPhone5 however, and was very glad for that.
When thinking about a smartphone, I care more about it's usability as a phone, then as a music/podcast player, and only finally as a media device. Am I out in left field here?
A bit, yes. My phone (n5) is used more as a web browsing device and video consuming device than it is as a music player. It is definitely the screen size. My previous phone (HTC Desire, about the same screen size as an iPhone <6) was pretty much a phone plus ipod.
I am still in 2 minds about my iPhone 6. It is much nicer in most ways than the 5. The extra screen real-estate really helps but at the cost of single-handed operation and comfort when bending at the hips.