Thing is, Rhaenyra getting fat is actually a minor plot point from the source material. Her stepmother Alicent Hightower gives birth to her four half-siblings and is almost a decade older than her, but stays slender and graceful into middle age. Rhaenyra, who had lost her own beauty by the age of twenty, resents Alicent for being both luckier & putting more effort into her appearance, to the point where it's literally one of several reasons why the two hated each other canonically.

Can't have Princess Champagne Feminist envy another woman for her looks or have any physical imperfection of her own though (and for all the bleating about how fat is beautiful nowadays, I suppose Sarah Hess & company don't truly believe that themselves after all), so not only has Rhaenyra been changed into someone who doesn't care about her stepmom's looks, but she's now portrayed by a more slender actress as well.