Some brands have bras that are actually supportive for larger sizes. I know Wacoal has some bras that actually work great up to size H (US sizes) . Beyond that though some really big ladies have had success with historical style corsets because they make your entire torso hold the weight instead of just some straps. Also unlike stage and Hollywood corsets, the historical ones are super comfy, but they have to be custom built for your exact body and they have a custom item cost.
Mind you she was most likely not formerly given etiquette lessons due to being an illegitimate child, that and she is often doing everything that no noble/royal lady would even think of doing and with her eccentric personality, i would expect that she would do some sort of eccentric poses. The arm over her chest to reduce the bounce as she has a large bust and considering the fact that they're not in a modern timeline so they don't exactly have modern clothes nor would they have thought to use a bra, she is most likely wearing a chemise since considering she is still a princess, is also most likely wearing a corset which doesn't assist in reducing the bounce.

Does it bother/ confuse anyone else when they illustrate the FC's in these ridiculous dynamic poses? Ain't no way in hell her leg is gonna be like that, especially as she's landing from a fall that high! And what's with that one arm covering the chest when they're running all the time? The ONLY time I ever ran like that was in gym class when my bra strap broke and I was trying to keep my boobs under control and from smacking me in the face!