After much questioning of reason, I was able to get a clerk to make a venti size Vivanno for an extra dollar.

I don't understand why the Vivanno is only offered in one size.
4 reasons off top of my head:

They are new at it. Smoothies have HIGH food cost compared to coffee. They used to sell $5 cups of coffe that cost em $0.25 to make. By offering smoothies, they are selling a $5 smoothie that costs $1.30 to make.

Smoothies have a lot of waste because you can never get all the smoothie out of the blender jar. There is an equal amount of waste on a large smoothie as a small smoothie, but more money to compensate for it with a larger smoothie.

Making small smoothies is difficult. It'd be like cooking a 2 ounce medium rare steak perfectly. Easy to burn/overcook, or undercook because you are scared of burning it. Its really hard to get a small portion correct. You either get chunks in it, or it is too thick and won't blend.

You'll buy it any way. Why sell you what you want for $3.50 when you'll pay $5 for something you don't necessarily want.
It's probably cheaper for them to do it that way, they may also make more money on it that way.

Anybody think that these days with all these silly sweet dessert-type drinks, that Starbucks is a lot like an expensive Dairy Queen?
Our official anwser is that since we use a real banana and we can't get growers to make quality half bananas, we can only make them in one size. So we opted for the grande size not to huge not to tiny.