In the base form, or what species you originally were, you can only grow in size by traditional methods. This was initially used to try and prevent any... weird art of the shapeshifters in the game. This does explain why Rosa is like she is after the whole becoming one thing, to make it easier to turn limbs into tools like weapons. Same goes with the Ancestor, but he is an exception to all rules, as he's the last defense against Peisma. So he can do literally whatever. I'm sorry to tell any of you horny people, when any form of shape-shifting happens the hosts body turns into the substance from which allows them to do so. And when changing limbs the substance detaches from the host and goes to the limb being transformed. When changing shape entierly it disperses and comes back together in the form they wish to turn into. I am trying to limit as many weird things about this take on shape-shifting as possible.