Vacuum Secondaries

moper said

The "right" size carb will vary depending on the use of the engine. That's why 600, 750, 850, and 950+ may be the "right" carb. One needs to know how to tune them. Then match the combo and use with the size. The typical math basing cfm on displacement and rpm required really has very little to no bearing on sizing a carb. Especially when some carbs are dry flow rated, some are wet, and 2bbls are tested differently.

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Exactly. Those are approximate numbers used the marketing department.
Some general trends related to sizing
Smaller primary venturis and throttles naturally produce higher signal both for the idle circuit (vacuum dependent) and at the main (velocity dependent).
Higher manifold vacuum tends to produce better fuel distribution amongst the cylinders. The better the manifold distribution, the less important this is.
Big venturis and throttles produce the least restriction and have potential to produce the most horsepower during high air flow demand. The better the cylinder to cylinder fuel distribution is, the more this can be taken advantage of. Droplet atomization size, etc can also play role.