650 dp vs 750 dp in a 408

The slightly greater restriction at high rpms could also be helping fuel distribution.

Also vaporization and homogenization as well amongst other things

If the demand is the only thing that matters a 1000 cfm would be perfect for every vehicle under 500 hp...even a slant six!
Carburetor design and operation are more important than size.

I agree. The engine wants what it wants and has to operate over a wide range of rpm which is where the smaller carb will shine. Also different designs and booster types produce different fueling styles which some engines prefer over others.