The figure. During the Regency being thin was not fashionable. It was the exact opposite. As you can see by the portraits, plump enough so that one’s collar bones were not showing was what one wanted to be. Curves were to be desired, not something to be ashamed of. Rounded faces were fashionable as well. Notice that many of the ladies had hints of double chins.
Naturally, there was a reason for that. If one was thin, it was assumed she could not afford enough to eat or that she was in poor health.
She must have good posture, something she would have been trained in with use of a board strapped to her back (backboard). Long stays also aided posture.
Her shoulders should slope. Her hips should be wide (this was thought to aid in childbirth).
This is Sally Jersey.
Emily Cowper
Mrs. Drummond-Burrell
ellaquinnauthor
Ella Quinn ~ Author 
Thank you, Ella! I grind my teeth every time I read a Regency in which ladies with slim figures are lauded as Diamonds of the First Water when they were actually disdained for being so clearly underfed and therefore not up to scratch! Even Marie Antoinette, though well before our time, was jnitially vilified fir being slim. So she ate cake-and lots of it to acquire the desirable plumper form demanded of ladies of high birth. That fact has always put quite a different connotation on her ‘let them eat cake’ which modern day readers assign quite a different, and less admirable sentiment to.
You’re welcome, Kalinya!