You know, symbionts may be smaller than they look during the joining. There are all kinds of real marine lifeforms on Earth that can pump water into their bodies to swell up to many times their original size. Then, once they expel the water, they smoosh down all small again. Maybe symbionts naturally inflate before being removed from the pouch to facilitate movement and to protect themselves from the air. Then they become much more amorphous once inside the host body.
