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.

Or big-looking shot glasses that have thick walls and bottoms. the glass looks bigger, and it's full... must be more in it.

To the magical thinker, a small glass filled up has "more" wine in it than a large glass half-full.

This is why we should be eating off small plates at home and buying the small donuts instead of the regular sized one. You get more satisfaction for the same sized meal or equivalent satisfaction for a smaller one.

I admit to using smaller forks and smaller plates when I'm grabbing a quick meal for in front of the PC - makes the food look larger, and it's easier to handle, too.

Beyond just the magical thinker, I'd assume the restaurant didn't use both size glasses at the same time so no easily-available means for comparison existed.