diminutize

To put (a word, name) in a diminutive form.

[…] her name is Concepcion, diminutized: Concha […]

[…] Marie Antoinette is referred to in the literature of the Revolution as the “Widow Capet”—a title containing no name of her own or any she herself had ever used. Elsewhere her given name is diminutized as “Toinon” and “Toinette.”

To make (someone or something) appear smaller (often in a figurative sense).

You must remember that it takes two to make a bargain. On your side you look through a telescope so that it will magnify the value of your holdings, while the buyer, on the contrary, looks through the reverse end of the instrument so as to diminutize it as much as possible.

1986, Jim Godbolt, All This and Many a Dog, London: Northway, revised edition, 2007, Part Two, Chapter 2, p. 140, The lady owner came out of the store, snatched the ticket from the windscreen, flung it to the ground and screamed: ‘You silly little man!’ at the warden, who was completing his entry of the offence in his book. He was of average height, but the rich invariably diminutize those who upset them.