I've noticed that at most McD's as well. The iced coffee drinks aren't filled to the top either and the sandwiches are smaller though it seems that the dollar ones have shrunk the most.
There's no way any McD's can be loosing $$ or they wouldn't stay in business. The drinks are cheap to make, think about how many they can make out of a squirt of syrup and some ice.
Just my thoughts on the subject.
Now I'm off to get an iced coffee! :) :)
~Deb~