The high-end ARM chips have gotten so powerful that they are even beating Intel's "netbook" chip, the Atom, which used to be a lot more powerful than ARM chips just a couple of years ago. But this also means a big part of that performance is not needed for smartphone low-end tasks such as receiving notifications, using the UI, and other background tasks.
Using a relatively complex chip to run these tasks may not be necessary, and may use too much energy. This is why ARM has come up with the idea for big.Little, which pairs a high-end ARM CPU like Cortex A15 with a low-end one like Cortex A7, which also happens to be a CPU that is used for low-end and mid-end devices this year, and probably for a couple more years after that, too, in ultra-low-end smartphones.
ARM says that when combining Cortex A7 with Cortex A15, you get half the power consumption compared to using a Cortex A15-only chip, and that's only for using an extra 2% or so of the die size, considering Cortex A7 is so tiny (0.5mm2). This may be the best way to deal with the chips in the future. Use a very low-power chip when you're doing mundane things, like checking e-mail and so on, but take advantage of much higher performance (about 2.5x more) when you want quick loading of web pages or want to play a visually impressive game on your mobile device.
ARM says that a lot of manufacturers are interested in the big.Little design, and we should see more of them adopting it, soon. So far the main chip making big.Little popular is Exynos 5 Octa, which should appear in the "second wave" of Galaxy devices in the second half of the year. So expect it in devices like the next-gen Note 3 and Note 10.1, and probably some other tablet as well (that's meant to be kind of a Nexus 10 successor meant for the holiday season).
Next year we should be seeing Cortex A53 and Cortex A57 continuing the same big.Little path, but on the ARMv8 64 bit architecture. A lot of companies are adopting them not just for mobile, but also for servers, as ARM chips are finally getting 64 bit support and now have enough performance for "micro-servers".