An orthopedic surgeon at 1010 Park Place discusses how aging is "bone deep," leading to muscle mass loss and volume shifts in the gluteal region after age 50.
For information on how mature and larger bodies change over time, several lifestyle and medical sources provide insights:
While there is no single academic or formal research paper specifically focused on that exact description, there are several publications, photo ebooks, and articles that explore mature body types, "BBW" (Big Beautiful Women) aesthetics, and the aging process of the female body.
There is a series of photo-focused ebooks titled which has dozens of volumes including: Mature BBW No. 51 (280 pages) Mature BBW No. 61 (263 pages) Mature BBW No. 71 (219 pages)