The difference between Apple and Windows is one of 'closedness' and 'open-ness' or 'breadth'. Windows is a large and bulky operating system because it has to support a massive amount of potential hardware configurations, because it has to be able to run legacy software (many businesses use bespoke packages produced in the early 1990s still) and needs to support programmers using different suites if it's to maintain its appeal to a large range of users (because these users can only have their needs met if there's a large dev community on Windows). It's a myth that the the OS-X kernel is any more efficient than the 'true' Windows kernel. The Win6 kernel is about the same size as the OS-X 'core'. Because it has to support a large range of software and hardware, though, Windows becomes less efficient and less secure as a whole.
Apple, by contrast, offer a small range of software, integrated into the OS, that means the operating system has to store less redundant data. Apples run on a small range of hardware - as you'll note with a hackintosh, you have to seek out specific hardware to work with OS X. That's not just because market share prompts developers and manufacturers to build with Windows in mind, but because OS X isn't built for anything but the iMacs it ships on.
Thus the advantages and disadvantages of Windows. Windows is very 'open' - it supports a large range of software and hardware, and is ubiquitous. This means redundant portions of the OS and security flaws, however. Apple is 'closed'. You get a speedy and stable OS, but you can only run it on (expensive!) Apple hardware (unless you steal the OS / break the EULA on the software you own, as per a frankenmac), and your options as a user are extremely limited (Macs don't break... because they don't let you break them). You get to do what Apple has in mind, but if you have specialist needs that the OS doesn't cater for, you may be better off with Windows.