The antonyms for the word "outweigh" are "underweight", "underbalance", "underestimate", "underrepresent", "underemphasize", "outbalanced", "outmatched", "outnumbered", and "outcompeted". These words have opposite meanings to "outweigh", which means to have greater importance or significance than something else. As an example, if you say that the disadvantages of a particular decision outweigh its advantages, then you mean that the disadvantages are more significant or important than the advantages. In contrast, if you say that the advantages of a decision underweight or underrepresented, then you mean that the advantages are less important or less significant than the disadvantages.