Yotsuba needs the energy to grow, Usagi doesn't need to get any bigger. You'll end up like Fuuka!

[See, that makes sense. Now someone get her those cupcakes. Curse you glass and your getting between Yotsuba and her enemies. Her very delicious new enemies.

Wait what was it they were doing?]

... No time for eating cupcakes Usagi, there's buried treasure to find!