Was the Delhi Durbar tiara altered by the time the Queen Mother allowed it to be exhibited? It is still a circlet, just seems to be wider at the top. I wonder how that was done. It looked much larger on Camilla than on Queen Mary or even the petite Queen Elizabeth.
Sartorial Splendor says the original diamonds were given to Queen Mary by deBeers, and she had them first made into her tall looped tiara, which was later dismantled for the Delhi Durbar diadem. That history explains why Queen Elizabeth wore it in South Africa.