This sounds very odd. "I want less soup" is very abrupt. "I don't want so much soup" a bit less so. "May I have a smaller portion of soup (please)" would be marked wrong, but would irritate your host less.
Duolingo is a lesson in translation, not manners :D But jokes aside, sentences are probably automatically generated to some degree by having sentences like "Eu quero menos <noun>" and then using inserting words marked as nouns.