Grant McCracken has posted a nice tale about doing ethnography at McDonald's to figure out why people order smaller drink sizes in the drive-thru than inside. And in the process, provides a nice concise explanation of a research approach.
For what it's worth, my guess is 1) cup-holder size or 2) the drink gets hot before you finish it. (The reverse being true with coffee, of course -- if the air conditioning is blowing on it, it gets cold before you finish it.) 3) Possibly people think they will not get the drink finished before they reach their destination -- after all, that's why they're in the drive-thru, right?
However, the real point here is using thoughtful research approaches to figure out otherwise intractable mysteries of human behavior.
Postscript August 16, 2005
The comment list on the post linked above is an amazing testament to homo sapiens desire to
solve problems. People are still posting about the McDonald's soft drink issue, and I have to admit, it's still bugging me ... my latest observation is
that you can see the size of the cups when you are inside, but there
are no cups to observe outside.
The other telling thing about this conversation is that it really highlights the importance of actually doing the research, not just thinking about a problem. Because when you actually do the research, you see many things that are not readily apparent from just thinking about something. (I may have to start hanging out at the drive through just to prevent myself from going bonkers trying to figure out the one correct answer.)