I've never owned a Sammy, but I think the reason the Sammy guys opted for the larger MC WAS because the Sammy's were small vehicles to start with but.... once you start adding Toy axles, taller tires, bigger and heavier bumpers, sliders, etc, suddenly the Sammy wasn't so "small" any more and they needed the additional volume and pressure to reliably stop their "bigger" Sammy.
The 2nd Gen trucks come from the factory with decent MCs in them because they were "bigger" trucks to start with.