I thought that no one had worn the Mitre after Bl. Pius IX, but we can see that Pope John Paul II wore it once. Well, he had to wear something that had to go with the Eastern Mitre-crowns.
The Immaculate Conception Mitre seems somewhat smaller (compared to the ones worn by Pius XII & John XXIII). However, as seen in this picture of Pope Pius IX, it seems he had a preference for somewhat smaller Mitres (but not ugly like the modern ones). (The above is supposed to be a picture -not a painting- that was later retouched, but I cannot prove it).
This picture (of vestments that belonged to Bl. Pius IX) again shows a beautiful Mitre that was smaller than the ones worn by Pius XII and John XXIII.