Apple's not one to give details on the amount of RAM that ships with its iOS devices. Xcode 7 GM asset catalogs have revealed that the new iPhone 6s has 2GB of RAM, double that of the iPhone 6 and the iPad Pro, Apple's powerful new tablet aimed at enterprise, has 4GB or RAM. This is double the 2GB of RAM that's included in the iPad Air 2, which was last year's flagship.
More RAM is necessary for smoother multitasking and faster access to apps. iOS 9 will be bringing various split-screen and multitasking features to Apple's mobile OS which will only work on the iPad Air 2 and the new iPad Pro because these features require more RAM capacity to work smoothly.
Source: 9to5Mac