Size of order is irrelevant.
This is about efficiency.
They don't take the order until you get to the window. Then you have to sit and wait for them to make it and hold up the line.
When I call it in, as I did today, when I get to the window they hand me my order and I pay and leave. Faster for me, and faster for everyone in line behind me.
It's frustrating when I pull up behind someone who has four people in their SUV and they are ordering at the window and asking a bunch of questions about the menu because even though they have already passed by a large menu in line, they still have zero clue what they are doing when they get to the window.
This isn't hard. It's just drive-thru ordering.
As I said, calling it in ahead of time is what smart people do.