The jet/orfice size should be for the correct A/F ranges throughout the RPM band. If that means it's done while upside down in a trash can to get right, so be it. :)
At full thottle, provided same size throttle bores, you would like as close to a 50/50 distribution from primary and secondary bores. Putting smaller jets up front with a rod stuck in it may cause distribution to become unbalanced. Having the front feed 40% or less and the rear 60% or more will not be a good set up and cause rich back cylinders, lean front, even though it all drop into the same mixing hole and be a 100% number.
As Tony stated:
The metering rod is the reason the primary jet is larger. If someone told you that was wrong, you need to do some math to calculate surface area of the opening. The amount/area a rod takes away from the opening is a big factor in why you usually have a larger jet up front. The rod is down in the jet all the time, even at WOT. That's how it meters for LESS fuel when you aren't on it.
Grab a book on how carter or AVS style carbs work, it will help explain the design/methodology.
