Judging from last year 5.5 to 4.7 inch sales. The regular iPhone 6 outsold the iPhone 6 Plus by around a 3:1 margin. So every 3 phones sold is the 4.7 inch iPhone compared to one 5.5 inch iPhone.
Apple simply doesn't produce enough 5.5 inch models especially at the beginning of the cycle to meet demand. But once demand catches up to supply Apple still sells more 4.7 inch iPhones. Biggest factor is the price. The 4.7 inch iPhone is just cheaper