In the actual game everyone has a default class model that's modified. For females there's a bust size change. Byleth's modifier is 200%. One of the highest in the game. Only one higher is Manuela at 225%
The summer banners always have the best art. Byleth here looking like her spine's gonna break but she's showing us the goods, so it's fine.
Rhea looking good, too. Heroes always delivers with the art.
For the record, that pose is entirely possible even though the way her torso is twisting may seem unrealistic -- try doing it yourself, actually. If this were drawn from another angle it'd probably look perfectly fine.