Mac OS X 10.5.9 Snow Leopard uses FEWER resources than Mac OS X 10.5.8 Leopard.
Ditto for a couple of other version of the operating system.
I don't think telling your customers that "Windows is fat and bloated, so we can be, too" is exactly what they want to hear.
"It's how computers and software work" is not entirely true. Look at how Firefox, Safari, Opera, and other pieces of software get faster, smaller, or both with each iteration. Even Windows7 is supposed to be less of a pig than Vista. There goes your Microsoft alibi.