Why is a tall coffee the smallest size?!

because their hipster ******* trying to be original

Assuming that you are speaking about Starbucks, tall isn't the smallest size.

There is a size called short, which is the smallest.

Most people only ask for a short cup of espresso or cappucino because you aren't supposed to drink a huge cup of it.

You can ask for any Starbucks drink in a short cup which is cheaper and fewer calories.

With olives, 'giant' is pretty small.
It's all advertising and promotion trying to make small look bigger, eh?
.

That's the way our society has evolved!