1. The first iOS "phablet" only happened in 2014, and was very late to market. This explains why phablet growth in 2014-15 is explosive, but does little to predict future growth.
2. We are now entering the second year of no significant new small phones being released for either iOS or Android. Of course small phones would fare badly when nothing good is for sale!
The best-selling iPod was the Nano, by a very large margin. The best-selling MacBooks were the 11" Air. If there is any truth to a "6C" coming out this year I expect it will do extremely well. (Unfortunately the days of the perfect 3.5" phones are likely behind us...)