From what I've heard the silk was meant to be a little crumply like linen - it adds to the fabrics character. It just got a wee bit too rumply in the coach
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Looking at the dress it seems to be a pastiche of styles from various periods: The colour of the fabric is similar to a dress worn by one of the Granddaughters of John and Sarah, the Duke and Duchess of Marlborough, Lady Diana Spencer, later Duchess of Bedford. Diana was the daughter of Anne Churchill and Charles Spencer 3rd Earl of Sunderland. There is a portrait of her by Charles Jarvas at Blenheim Palace - the family tried to marry her off to another Prince of Wales - Frederick Louis, son of King George II.
https://en.wikipedia.org/wiki/Diana_Russell,_Duchess_of_Bedford
The volume of the skirt and the crinoline are 1860s and this was meant to be inspired by Vivian Leigh in Gone with the Wind, along with Dior, Valentino, and Norman Hartnell. That and a big puffy skirt just says "princess" regardless.
The sleeves are half 1890s and half 1630s - both Edwardian and Stuart. An unintentional reference to the brides ancestry maybe? The rounded, relatively high (at least to my eye) waist also seems inspired from the Jarvas portrait, but also looks quite Van Dyke to me along with the sleeves as well. The Frills and relative height of the neckline and the lace paneling on the bodice are dead ringers for Edwardian influence too.
It actually in terms of size, scope and poofiness reminds me of Queen Alexandra's wedding dress oddly enough.
The 25 foot (7.6 m) train I think is still the longest for a royal wedding - previous record holder was Queen Anne Marie of Greece who had a 20 foot (6.1m) long train.
You also can't go wrong with a sparkly veil either - makes a change from the usual choice of the moth eaten table cloths that pass as antique lace veils. It also links Diana's dress to that of the Queens, who also had a considerable amount of sequins and spangles on her veil, train, and dress.
A big dress was needed to fill a big building, so the scale made sense for that reason as well as the fact that Diana was a tall, big boned lady, it made her look smaller next to Charles. It also needed to look good on camera, so the colour and the sequins did help with visibility and ivory was less jarring on camera than pure white and worked better with Diana's skin tone.
This monster has always been a sentimental favorite for me and although, casting a critical eye over it I can see how it does not work (waist line is unforgiving, a v-point would have been more flattering, shrink the sleeves a little, and make the neckline a little lower and more defined and less frilly). But it did stick in peoples minds, Diana looked the part of the fairy tale princess in it, and it set the dominant trend for wedding attire for good or ill for the next two decades.