plump up
1. To become very plump, chubby, or rotund. Soak the dehydrated mushrooms in a cup of water for an hour, until they have plumped up and the water is nearly gone. I began plumping up almost as soon as I became pregnant.
2. To make someone or something plump, chubby, rotund, or fuller in shape. In this usage, a noun or pronoun can be used between "plump" and "up." My kids always come home for the holidays looking so skinny, so I always try to plump them up a bit. It has come to light that one of the biggest poultry producers in the country has been plumping up chicken meat with water to artificially increase their weight and subsequent price.