In theory this boob size could be justified if her tits were stuffed with eggs/cum, but that might be a bit of an overkill.
Or how about this: Dryad tits are stuffed with special dryad milk that slowly regenerates as the player walks around and can be used as a healing potion in combat? (And may be to cure void infections in events.) Also depending on how big her tits are the amount of times she could heal herself in combat through the use of her own milk will also increase because she can carry more of this milk in her tatas.
Heheh, leave it to me to come up with some kinky shit that's also practical.