"Sure, but then running application state includes few shadow framebuffers which needs memory proportional to the size of raw data on your screen."
Maybe the misunderstanding here is not differentiating between the memory requirements of running vs. stopped applications, and the associated state transitions.
It seems that if many apps (which ones?) use large pixmap caches during operation, *and* these are all retained in RAM while the apps are stopped, *then* perhaps overall RAM consumption can be said to be proportional to framebuffer sizes.
If many apps do not do not keep large pixmaps during run, or if they jettison them upon an application stop, then no.