The mechanism is that human brains are really, really bad at judging volume. A small glass full to the brim looks like it has more than a large glass that's hal

The mechanism is that human brains are really, really bad at judging volume. A small glass full to the brim looks like it has more than a large glass that's half full. Bars often use a related trick, which is tall thin shot glasses - they look like they have more, even if they have less, compared to wider glasses.