Sorry, just a quick reply here. The pressure in the venturi may drop as flow is accelerated, but the manifold pressure gauge is measuring the pressure in the inlet manifold just before the fuel/air charge enters the cylinder, it has already passed through the venturi.
The bigger and heavier and fatter the fuel/air charge, the more pressure in the inlet manifold, the more power produced by the engine.
Hope that makes sense for you.
Dragman