a Qjet isn't that great, hence they stopped using them on production cars in the 80's. That being said it can be tuned to perform much better or much worse for a given application.
Most carbs can be decent for mileage (none will touch a good EFI setup) but its all in TUNING.
A bigger carb can deliver better mileage than a small one IF the bigger carb is tuned better. If the smaller one is tuned better then it will perform better.