James,

Think of this in terms of Ohms Law. Think of the pressure as voltage, volume as current and resistance to flow as resistance.

Now, think of the vac as able to produce very low voltage(pressure,) but high current( flow volume.)

When you choke that 2 1/2" hose down to 1/4", you have raised the resistance. With the pressure being the same, there is not enough pressure (voltage) to push any volume (current) through the circuit.

Hope this helps.