this is only half of the truth; it is very true that current macs have plenty of memory, but that's not because mac software eats it for breakfast, it's because one can benefit from it. because mac is a unix, there can be as much memory as your wallet permits, and it can be all in use if you like.
windows, however, lives still in the stone-age where people was amazed to see a computer with a whopping 16MB of ram. windows handles the memory in such a way that it tries to keep maximum amount of memory available to apps the user may like to run in the future, in other words, it keeps the memory free and the contents that should reside in memory, it flushes down to the virtual memory (hard drive). that's the reason fewer windows users buy a lot of memory, simply, because it cannot be used efficiently.