micromania
/mʌɪkrə(ʊ)ˈmeɪnɪə/Chiefly Medicine and Psychiatry. Now rare
noun
The delusion that the body or part of it has become abnormally small. Also: a persistent or exaggerated tendency to belittle oneself or regard oneself as unimportant.
Origin
Late 19th century; earliest use found in The New Sydenham Society's Lexicon of Medicine and the Allied Sciences. From micro- + -mania.