Steven Sinofsky explains in his latest blog entry how the memory usage has been reduced in Windows 8

The runtime memory usage of Windows 8 is an important factor in determining the Windows 8 system requirements, as well as the broadened spectrum of devices that will host Windows 8. As you know, we’re delivering the complete Windows 8 experience on SoC-based devices characterized by low power consumption. This makes it even more important to leave lots of memory available for multiple concurrent apps and to sustain the overall responsiveness of devices.

Something that might not be obvious is that minimizing memory usage on low-power platforms can prolong battery life. Huh? In any PC, RAM is constantly consuming power. If an OS uses a lot of memory, it can force device manufacturers to include more physical RAM. The more RAM you have on board, the more power it uses, the less battery life you get. Having additional RAM on a tablet device can, in some instances, shave days off the amount of time the tablet can sit on your coffee table looking off but staying fresh and up to date.

  Reducing runtime memory in Windows 8