Both iPhone 6 and iPhone 6 Plus come with a high resolution display that give apps real estate to show more content, images, text and so on.. Developers, however, need to update their apps to take advantage of the larger screen. Unoptimised apps run in scaled mode, where everything looks much larger than it should be.
A new recently released jailbreak tweak called ForceGoodFit, this tweak forces unoptimised apps to run at native resolution rather than in scaled mode. This is great because a lot of popular apps like WhatsApp, Google Maps etc. still haven’t been updated for the iPhone 6 or iPhone 6 Plus.
The tweak is very similar to FullForce, another tweak that forced unoptimised 3.5-inch apps run at full resolution on the 4-inch iPhone 5, iPhone 5s and iPhone 5c.
You can install the tweak for free from Cydia, and enable it in Settings > ForceGoodFit. If you don’t see all the applications in that list, you might also have to install AppList.