Understate? It depends on the context, I'd say.

To downplay Example: I didn't want her to worry about me, so I downplayed my health issues.

Hi Ethan, you can check out this resource: http://www.thesaurus.com/browse/exaggerate

I saw that wrong as well, I was thinking synonyms when he clearly asked for antonyms which means opposite, big - small, exaggerate - downplay, cold - hot, etc. Easy mistake.

amplify, distort, emphasize, fabricate, falsify, heighten, inflate, magnify, misrepresent, overdo, overdraw, overemphasize, overestimate, boast, boost.