Sorry to be slow getting back here but it is a detailed story, which I will be posting in the RCMB Forum. I'm not using the word "resizing" which appeared only in the royal family tweet. See Karen's message that opens this thread. "Fitting" or simply "preparing" are better words. A few points here only to head off too much speculating.

1. St Edward's Crown was used unaltered for Elizabeth II because it fitted her.
2. It has a sort of skullcap or helmet to fit the wearer much like the leather skullcap of the Guards Bearskins.
3. The solid gold band has been cut through, on both sides, in the past.
4. It was originally made for a bewigged head with a wider base and probably straight sides.
5. After the cutting it has the waisted shape we know today.
6. The cutting might have been part of the work for either or both the Edward VII 1902 and George V 1911 coronations.
7. Edward VII planned to us it for his crowning but George V was first to do so after well over a century or more.

I do have to do more work and will supply my sources in the RCMB Forum.

Edit: I need to add re point 4 that a small booklet souvenir "the Crown Jewels" first published 1953, second impression 1970 says "...it was made to surmount the long, flowing black curls - not, at that time, a periwig, but the king's own hair..."

Edit 2: the source for "bewigged head" in point 4 was EF Twining's tiny booklet "The English Regalia and Crown Jewels in the Tower of London," undated edition possibly late 1940s.