You should read over the Mavericks DP threads. Users there are reporting much better memory management. My point is due to these changes in Mavericks it will need less memory than say Mountain Lion and not more, so the old axiom about successive OS updates always requiring more memory does not necessarily hold.
Much as I'm sure there are valid opinions in such threads I'm wary of treating them as any kind of science...
Anyhoo, you're right, the axiom does not necessarily hold in this case.