If you think about it:
No sponge = limited surface area of filter media for bacterial growth = high flow is required
With sponge = more surface area for bacteria = lower flow over filter media is required
With a lot of sponge = a ton of surface area for bacteria to grow = very low flow over a large amount of bacteria to keep ammonia/nitrites processed.
Using a sponge to reduce water flow seems to be a direct relationship that takes care of itself, i.e. the size of the sponge will slow down your flow in a proper amount to maintain the cycle while providing more surface area for you BB.
Think of an air powered sponge filter: Large sponge, low flow created by rising air bubbles, large surface area for BB to process waste.