The perfection of Diction is for it to be at once clear and not mean. The clearest indeed is that made up of the ordinary words for things, but it is mean... A certain admixture, accordingly, of unfamiliar terms is necessary. These, the strange word, the metaphor, the ornamental equivalent, etc., will save the language from seeming mean and prosaic, while the ordinary words in it will secure the requisite clearness. What helps most, however, to render the Diction at once clear and non-prosaic is the use of the lengthened, curtailed, and altered forms of words.
This extract is from Aristotle's Poetics and as you may have noticed they articulate his idea of an ideal Poetic Diction. I am having trouble understanding the last line and would greatly benefit if one could give an example of pairs of words that have opposing attributes (the attributes being lengthened and curtailed).