Small paintings that accompany text in a manuscript or printed books are often called miniatures, not because they are small, but because a red lead pigment, called minium in Latin, was frequently used to decorate the initial letters.
"An independent illustration, as opposed to a scene incorporated into another element of the decorative scheme such as a border or initial" (Brown 86).
D&D was originally an outgrowth of fantasy wargaming, principally Chainmail. The Original Dungeons & Dragons bore the subtitle, "Rules for Fantastic Miniature Wargames Campaigns Playable with Paper and Pencil and Miniature Figures." Initially, fantasy wargamers used figures designed for historical wargaming, but eventually, new lines of fantasy figures appeared.