Eurogamer had an interview with Microsoft engineers about the CPU/GPU of the Xbox One console, you can read it over here.
Digital Foundry: CPU-side, I'm curious. Why did you choose eight Jaguar cores rather than, say, four Piledriver cores? Is it all about performance per watt?
Nick Baker: The extra power and area associated with getting that additional IPC boost going from Jaguar to Piledriver... It's not the right decision to make for a console. Being able to hit the sweet spot of power/performance per area and make it a more parallel problem. That's what it's all about. How we're partitioning cores between the title and the operating system works out as well in that respect.