PassengerSpawnMethod "smart" doesn't reduce memory footprint?

Our Rails 3.2 application (Ruby 2.1.5) sits behind Passenger (4.0.59) and uses the default spawn method, which the Passenger docs stipulate is "smart".

Smart spawning is claimed to shave about 33% off the per-process memory footprint.

I was curious how much benefit we were deriving from using smart spawning so I modified our config to specify "direct" spawning.

However, after switching to "direct" the memory footprint of our processes didn't increase like I expected. This seems to indicate we aren't actually deriving any benefit from smart spawning.

Any theories about why this would be the case?

Are we not in fact using smart spawning despite specifying it in the config file? Is "top" outpu (specifically the VIRT and RES columns) not a good way to measure per process memory usage?

Something else entirely?