It's a hassle on both the costumers and the crew to have all the menu items outside. People spend twice the amount of time in the drive-thru then inside. By making the sign smaller it would make the drive-thru quicker. Thus in getting more satisfied customers.
The menu inside would still contain all items, however the outside menu would only contain meals, the special items that would be being served (i.e. McRib), the 2 for 5, and all the deserts and salads. Therfore the customer still has many options but it makes it easier on the workers.