It's not that simple.Not really. Modern processors are much more powerful while consuming less power than previous versions. The smaller the fabrication nodes allow for less power consumption and heat generation.
The most likely reason for Apple to going BIG.little is that they were stuck with 16nm with the A10. They had to give up some power efficiency (the big cores likely 'leak' more power at idle) to make them more powerful than the A9 2 cores. The 'little' cpus had to be added so they can do the work while you're listening to music/taking a phone call etc. while being more efficient than the A9 while doing those tasks. The only disadvantage is the bigger die space is more expensive for Apple to make.
Higher performing cpu's like Core i7's perform really well at high clockspeeds, but still consume too much power at slower speeds as they were optimized for the higher end of the spectrum. Vice versa, you can't just take an A9/A10 and clock it to 4.5ghz and expect it to lay waste to Skylake as it was optimized for power savings.