I was shopping for phones today and physically they're all bigger than my almost five year old iPhone 4. The user interfaces are essentially the same as on my iPhone or my even older G1.

I did see a curved smartphone for the first time though, so I predict in 5 years phones will be semicircular, about 40% larger than they are today, and have slightly faster graphics.

Going totally off topic here, but if you're willing to try something different than iOS then have a look at the Sony Xperia Z3 compact. Small (by todays huge standards) but packs a good punch in specs :)