antonyms in sound symbolism study

I was interested in the use of antonyms in a study exploring sound symbolism. To quote a Quartz article about it:

Kaitlyn Bankieris, a cognitive scientist from the University of Rochester, and Julia Simner, a psychologist and leader in the field of synesthesia, showed participants 400 adjectives from 10 languages they didn’t speak: Albanian, Dutch, Gujarati, Indonesian, Korean, Mandarin, Romanian, Tamil, Turkish, and Yoruba. The words were broken up into categories by meaning: big/small, bright/dark, up/down, or loud/quiet. Participants heard the words spoken aloud and guessed their meanings.

Synaesthetes were better at the task than others, but overall both syntaesthetic and non-synaesthetic could guess the words at a better-than-chance rate, presumably because the words with 'small' meanings restrict the vocal space more and the ones with 'large' meanings open the vocal tract more--this is known as sound symbolism. Another article about the article can be found at Scientific American.

One assumes that if they asked people if the word meant 'small' or 'bright', they'd be back to chance. It's being on one or the other side of a scale that leads people to the right answer.