I'm not very new to reloading but have wondered about this for a while and I figured some of you "salty dogs" already have the answer.

Why does a parent case always shoot same weight bullets faster and tolerate heavier loads than necked down versions? Meaning, why does the .308 win shoot a 160 gr bullet faster than a 7-08 will? ...and why will the 7-08 shoot a 100 gr bullet faster than a .243 will? Obviously, the case capacity is basically identical before seating a bullet, so what gives?