take a fluid dynamics class and it will make a lot of sense as to why the venturi is designed the way it is, and also the way that inserting the fuel into the air stream at different points will allow more power or better performance. I also would say that the rating they put on their carbs may be more for marketing than from actual specs. But if you know how it works and have the access and ability to test them you will be able to pick the right one (Who is going to go buy 5 carbs to test though LOL).
Oh and IIRC then a smaller venturi may be on a larger carb and rated at a higher cfm because it could be that the air and fuel get to that small section they flow faster and therefore more gets into the engine than with a fuel toilet design. Also the design of the venturi will dictate wether the air is in a primarily laminar or turbulent flow and how it will atomize the fuel, this goes so far as to the design of the manifold when the fuel hits that it needs to be flowing well enough and not hit the bottom of the plenum like a rock. I hope this helps, if you need anything specific I have a fluid dynamics book and know how to use it and do the math for it and can try to explain some things in more detail.
-Josh
Oh and IIRC then a smaller venturi may be on a larger carb and rated at a higher cfm because it could be that the air and fuel get to that small section they flow faster and therefore more gets into the engine than with a fuel toilet design. Also the design of the venturi will dictate wether the air is in a primarily laminar or turbulent flow and how it will atomize the fuel, this goes so far as to the design of the manifold when the fuel hits that it needs to be flowing well enough and not hit the bottom of the plenum like a rock. I hope this helps, if you need anything specific I have a fluid dynamics book and know how to use it and do the math for it and can try to explain some things in more detail.
-Josh