There's definitely some cultural context missing, I know that Donguri means Acorn, but I have no idea what Sakuya means by saying it here. Which must be why it's gone un-translated.
Ahh! Onee-sama is flat.The flat one here is Flan!!Say something too, Sakuya!!Barely a gap...
Donguri no seikurabe ( どんぐりの背比べ , lit. "like comparing the size of acorns"), meaning a pointless competition between two equally unimpressive parties
