KDE do some caching (I don't know what the settings are in Kubuntu, but it is probably enabled).
It's like comparing Windows XP dans 7. 7 will max out your memory at anytime, but in fact use less tahn 10% more than XP. KDE do a lot of caching to make apps load faster. Memory don't need to be wasted. All modern OS do some caching to make the system feel faster. Of course Kubuntu is a terrible KDE distro and feel slower than Vista, but it's an other story. But please keep in mind that it does mather in a professional benchmark, your KDE stats are biased.
Only Gentoo with -Os in make.conf will give the real picture. At least you know that both DE have been compiled the same way, and not optimized with different settings.