[winterismute]

I have been doing exactly the same thing for years! But personally I think it also depends on, well, your physical attributes: being tall/big makes the backpack seem always small, and also makes it kind of invisible to the people in front of you (who are the ones that will eventually decide whether to check if your backpack fits into the limits or not). I think the same trick would not work for shorter/smaller persons. But I might be wrong of course.