Venti is the Starbucks name for their largest size cup, which holds 20 ounces.
Most people don't really care about the exact, specific volume of fluid, they just go by whatever the cup size happens to be called.
At least it is more descriptive than the grande or short cups, or the small / medium / large at other places (even if you have to know that venti is 20 in Italian to make the connection).
I doubt I'm the only obnoxious country dude who rolls into Starbucks and orders a "large". You can ignore their pretentious names, they're very accommodating.
It's probably more that you don't want a corporation to dictate how you describe (brand) the sizes of a drink where we have near universal language for that purpose already.
Too much experience with cattle and thus not longing to resemble them. One of 'm says 'moo' and soon the rest follows. Now just imagine a ${city_equivalent_of_field} full of city folk where one of 'm says 'venti' and you understand what I mean.
Most people don't really care about the exact, specific volume of fluid, they just go by whatever the cup size happens to be called.
At least it is more descriptive than the grande or short cups, or the small / medium / large at other places (even if you have to know that venti is 20 in Italian to make the connection).