According to Apple CEO Tim Cook, the 5.5-inch iPhone 7 Plus, released in September of 2016 alongside the smaller 4.7-inch iPhone 7, is the most popular "Plus" model Apple has released, outselling the 2014 iPhone 6 Plus and the 2015 iPhone 6s Plus.

Apple's Plus lineup has always had differentiating factors compared to the smaller iPhones Apple offers, but in the iPhone 7 Plus, the dual camera appears to have been particularly popular with customers. The iPhone 7 Plus features both a standard wide-angle lens and a 56mm telephoto lens, which can be used to improve photos.

It also features a "Portrait" mode that's used with the 56mm lens to create high-quality DSLR-style images with blurred backgrounds to highlight subjects, a unique function that may have drawn more interest.

Customers may also be getting used to the larger form factor in general. In 2014, with the switch to the 4.7 and 5.5-inch sizes, going from the 4-inch iPhone 5 line to the 5.5-inch size was a huge jump, but with two years to adjust to the new display sizes, it's not such a leap.

Cook said that Apple had underestimated demand for the iPhone 7 Plus, leading to supply and demand issues with the device that impacted sales. Despite that, Apple saw record iPhone sales during the quarter, selling a total of 78.3 million iPhones.