The story
From writ (“(archaic) written”) + large, from the poem “On the New Forcers of Conscience under the Long Parliament” in Poems, &c. upon Several Occasions (1673) by the English poet John Milton (1608–1674): “New Presbyter is but Old Priest writ large”; Milton was using the phrase in the sense “written more completely”.
Etymology from Wiktionary, CC BY-SA 4.0.