Bulk out is a phrase that implies adding volume or weight to something to make it appear larger or more plentiful. When it comes to synonyms for this phrase, some words that are commonly used include expand, extend, augment, supplement, and increase. All of these words describe a process where something is made bigger or more substantial through the addition of extra material or resources. Other synonyms for bulk out could include enhance, amplify, intensify, or fortify. Essentially, any word that suggests an increase in size, quantity, or quality could be considered a synonym for bulk out.