WordNet sense 1 (impaired by diminution):

lessened, vitiated, weakened

WordNet sense 2 (diminished in strength, quality, or utility):

impaired

WordNet sense 3 (made to seem smaller or less (especially in worth)):

small, belittled

WordNet sense 4 (made less in size or amount or degree):

decreased, reduced

WordNet sense 5 ((of an organ or body part) diminished in size or strength as a result of disease or injury or lack of use):

wasted, atrophied