The venturi is designed to meter fuel at a certain intake velocity range, go beyond that range and the venturi doesn't produce a proportional vacuum in relation to air flow.
You can only produce so much vacuum until you reach zero PSI, unlike pressurization which has no real limits...so when you approach that level of vacuum signal on the main jet, more intake velocity doesn't equate to proportional vacuum, which in turn doesn't equate to more fuel flow.
So in order to get that fuel flow you need to increase main jet size to compensate for the lack of proportional vacuum signal.
Sounds like jibberish, doesn't it?