The people on that subreddit only accept boobjobs that are "subtle", anything big and noticeable is automatically trashy to them, or "male gazey" as they say 🙄. The funny thing is that if she had naturals that size, I'm sure she wouldn't get nearly as much hate. There absolutely are models with natural boobs at her size and body proportions, so they're way off base in saying that they're too big for her body.
The OP also didn't quite do justice to Sienna's very pretty face either with that pic selection, they should've selected some of her modeling pics!