The iPhone 6 is likely to get a resolution bump to go along with its larger screen size(s). The iPhone 5s owns a resolution of 1,136 x 640 but as 9to5Mac points out, the next model could push that up to 1,704 x 960 which would result in a pixel per inch rating of 416 on the rumored 4.7-inch model and 356 on the larger 5.5-inch device.
The iPhone rumor mill is in full swing as we're probably 3-4 months out from the launch of Apple's next generation handset(s) with the majority of the discussion focusing on display size. All indications suggest Apple is preparing to launch two different models - both with larger screen sizes than currently available.
If true, it'll be a significant shift for Apple as they have traditionally been reluctant to play the "bigger is better" game. Since the original iPhone was released in 2007, Apple has only increased the size of the iPhone once - from 3.5 inches to 4 inches - starting with the iPhone 5 in 2012.
Even still, it's no secret that the market is moving toward larger phones and customers want something a big bigger even if it would be more difficult to control with a single hand.
Customer demand is only part of the equation, however, as they're very mindful of resolution bumps as it relates to developers. The rumored 1,704 x 960 resolution would essentially be a 3x bump over the "base" resolution of 568 x 320 and would allow Apple to retain the current Retina branding.