When the bottom piece of bread of your hamburger/sandwich is about twice as small as the top one, and only fits your thumb.

Leading to it sliding apart and making a mess.
Customer 1: I got myself a Big Mac, and I swear the bottom piece of bread disappeared..

Customer 2: Dude, it's the Big Mac Effect.

Customer 1: Damn it, now it's fucking everywhere..
by Eckpired January 08, 2011

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