Apple's (NASDAQ:AAPL) new iPhone models -- the 8, 8 Plus, and X -- are dominating tech headlines. However, a recent report from Newzoo, which tracked global smartphone usage through the end of July, revealed that of the 730 million active iPhones worldwide, the iPhone 6 and 6s were the most popular devices.

A chart comparing the market shares of iPhone models

Data source: Newzoo (September 2017). Usage tracked through July. Chart by author.

Newzoo's prior report, which tracked device usage through April, found that the iPhone 6 and 6s respectively controlled 21% and 18% of the market. The iPhone 7 and 7 Plus moderately grew their market shares since then, from 11% and 8%, respectively.

The iPhone 6 and 6s remain popular for a few reasons. They were the last models with headphone jacks, and their chassis designs and larger screens represented a big leap forward from the iPhone 5s -- unlike the incremental improvements in the iPhone 7 and 8.

Meanwhile, customers who were eyeing the iPhone but waiting for lower prices -- particularly in overseas markets -- bought the older models. These trends could be troubling for Apple, since analysts focus on the company growing its iPhone shipments every year.

Leo Sun has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Apple. The Motley Fool has a disclosure policy.