Blogs are reporting the news published by a Chinese technology website that the forthcoming iPhone 6 with a 4.7-inch display will have a battery capacity between 1,800 and 1,900 mAh, compared to 1,570 for the current iPhone 5s. The larger 5.5-inch iPhone 6 is expected to have a 2,500 mAh battery. Does this mean that the new phones will have a much longer battery life? Not necessarily. A larger capacity battery will be needed to power the larger screen of the forthcoming phones. In fact, large-screen Samsung models have even greater battery capacity than the rumored sizes for the new iPhones. The Galaxy S5, for example, has a 2,800 mAh battery. However, Apple's new A8 chip that's rumored to be in the new iPhones is engineered to be more efficient and to give longer battery life. It could well be that Apple will be able to increase battery life in the new phones while using a smaller battery than competitors. This, of course, has the advantage of keeping the phone thin and light.
There are always tradeoffs between thinness, lightness, and battery life. The Galaxy S5 has a significantly longer battery life than the iPhone 5s, according to testing by iPhoneArena.com. But it's also thicker. Apple always wants its devices to be as thin and light as possible. Yet they're still able to give very good battery performance due to their legendary engineering.
Apple almost never takes a step backward. You can generally expect their new devices to be thinner and lighter, with a longer battery life. One exception was the first-generation iPad with retina display, which was thicker and heavier than the iPad 2, but they made up for that with the iPad Air.
There's no way of knowing whether this report out of China is accurate. But inevitably Apple will increase battery capacity, and these reported sizes seem reasonable.