Next in "I can't believe they're not cognate":

"miniature" is not the same root as "minimum"/"minimal" etc, and comes from a (prob) Celtiberian root meaning cinnabar (mercury er sulphide I think)…

…which gets imported into Latin as minium. It's used as a red pigment so miniare = "to paint red"

…later this means "to illuminate a manuscript" because the prototypical form of illumination is writing key letters/words in red. And "miniatura" then is a noun form of that (cf. "picture"/"scripture")