The torso of the dress looks very small. Nutrition in those days didn’t build the kind of bone structure we have today. Here’s a brief bio of the lady, who was a Scot:
https://en.wikipedia.org/wiki/Jean_Ker,_Countess_of_Roxburghe
Remember, it is both wrinkled and probably shrunken from being in the sea. Also we don’t know if it was for the child or a thin elderly woman with osteoporosis. Do we know anything about the household she was going to? Did they have young women needing gifts of clothing? How old was the prospective groom? Among the nobility, marriages were made for political reasons. The contract was often signed when the promised were quite young. Sometimes the husband to be was even younger than the wife to be. In the western countries, I think they usually delayed marriage until puberty of the bride.
I visited friends renting a small cottage in Chincoteague, VA. The ceilings were very low, and my husband of 5’11” was able to walk through the doors OK, but the poor 6’2” guy kept hitting his head. The house was built before the bridge to the mainland was built in 1924. Prior to the easy access of new blood, the women were usually 90lb. and the men 120lb. They mostly bred among themselves, and occasionally a runaway, Army deserter or petty criminal would show up to stay and add a new surname.