Any Size

I don’t get this – please explain:
The sign did advertise
That cold-brew coffee costs a buck,
No matter what the size.

Iced coffee sounded good to me
So I proceeded in,
Where cups were stacked there waiting
For the filling to begin.

I asked the clerk which cup to take.
He said, “It’s up to you.
A small or extra-large will cost
The same when filled with brew.”

Of course, I chose the biggest
But what I don’t get at all
Is why any paying customer
Would opt to take the small.