1580s, "a reduced image, anything represented on a greatly reduced scale," especially a painting of very small dimensions, from Italian miniatura "manuscript illumination or small picture," from past participle of miniare "to illuminate a manuscript," from Latin miniare "to paint red," from minium "red lead," used in ancient times to make red ink, a word said to be of Iberian origin. Sense development is because pictures in medieval manuscripts were small, but no doubt there was influence as well from the similar-sounding Latin words that express smallness: minor, minimus, minutus, etc.
"on a small scale, much reduced from natural size," 1714, from miniature (n.). Of dog breeds, from 1889. Of golf played on a miniature course, from 1893.
late 12c., mycrocossmos (modern form from early 15c.), "human nature, man viewed as the epitome of creation," literally "miniature world," from Medieval Latin microcosmus, from Greek mikros "small" (see micro-) + kosmos "world" (see cosmos).
General sense of "a community constituting a world unto itself, a little society" is attested from 1560s, perhaps from French microcosme. A native expression in the same sense was petty world (c. 1600).
Forrþi mahht tu nemmnenn mann Affterr Grikkishe spæche Mycrocossmos, þat nemmnedd iss Affterr Ennglisshe spæche Þe little werelld. ["Ormulum," c. 1175]