I'm a riddle in nine syllables,
An elephant, a ponderous house,
A melon strolling on two tendrils.
O red fruit, ivory, fine timbers!
This loaf's big with its yeasty rising.
Money's new-minted in this fat purse.
I'm a means, a stage, a cow in calf.
I've eaten a bag of green apples,
Plath is comparing her big belly to the baking of bread. Yeast is used in bread to help it rise and have a nice, soft and fluffy texture…which happens when it is almost finished baking. Plath’s babies have grown even bigger and she is almost ready to go into labor. 

Metaphors