The word "under-sized" is most commonly used to describe something that is smaller in size than it should be. However, if you are looking for a word that is the opposite of "under-sized," there are several options to choose from. One antonym for under-sized is "oversized," which refers to something that is larger than normal. Another option is "adequate," which means that something is the appropriate size for its intended purpose. "Proportional" is another antonym for "under-sized," indicating that something is correctly sized in proportion to other related objects. Finally "ample" can also be used as an antonym as it means there is plenty or enough of something.