The Linux kernel is used in a lot of different devices, 
    this is true, but it's also a lot leaner than the Windows 
    one. Although efforts have been made to pare down what 
    Windows is, it's still got a footprint gigantically bigger
    than Linux.

I think Windows Phone 8 shows that this is not meaningfully true anymore. There's more work to be done, but the kernel on those devices are quite minimal, happily running on underpowered ARM devices with minimal RAM, and still leaving plenty of room for apps.