it's all about flow. smaller chambers and ports can create a very strong vortex that helps with scavenging. kind of the like difference between running in the dead of summer with 90% relative humidity and running in the winter in 30* weather.
http://hotrod.com/techarticles/48778/
http://chevyhiperformance.com/techarticles/41598/ - just about every head on the market is tested and the numbers are posted here