I saw a lot of shop/website offering bigbore carbu like WT-257 big bore
are there any difference when using big bore and standard carbu on the same engine?

from my knowledge, big bore is only enlarge the venturi which bring more air, does more air mean more fuel draw into intake tract?
with enlarging the venturi we change the pressure around venturi throat and less fuel will be drawn by the system, right? so how come big bore carbu make our engine run in better performance?

if it does, how to know what is the optimum size for venturi?
in this case I am running Zen G260 and G300 PUM