Whittle down is a phrasal verb meaning to gradually reduce or lessen. Here are some antonyms or opposite words for whittle down: increase, augment, add, multiply, enhance, enlarge, expand, grow, accumulate, build up, raise, reinforce, strengthen, amplify, mount, develop, widen, extend, proliferate, and intensify. These words represent actions that involve adding or increasing something. For instance, you can enlarge a picture or expand a business. You can also grow a plant or develop a skill. These words, therefore, carry a positive connotation unlike whittle down which suggests a process of gradual erosion or depletion.