My understanding about the floral headpiece was it was formed in the shape of a floral tiara.....to be removed by cutting from the diamond tiara she wore once vows exchanged.....
The reason Sarah's silk duchess satin gown appeared to fit her better is that it did as she did not lose the amount of weight the Princess of Wale's lost in the two weeks prior to her wedding to the Prince of Wales. Diana's gown, though lovely in the heavyweight silk taffeta especially woven for her gown due to width requirements for cutting the train and gown's skirt, was overscaled to her immense weight loss. The other thing to consider too is that the gown's style and the setting was very much on the designer's mind. The scale of St. Paul's is immense and to just show up at all, the gown had to be designed on a grander scale. Although Westminster Abbey is very large, the doorways through the church to the altar, Sarah's gown barely fit through, and in fact, her veil almost tore, having been caught on the wood on her way to Andrew.
The other thing to consider when comparing the two gowns is also the age of the bride....Diana was very young and very naive.....and so the design reflected her mindset at that time. While I loved the satin of Sarah's gown, I appreciated the design of Diana's and the reasons for the choice.