One night, I had a dream where I was walking in a niche convention (on like ball-jointed dolls or similar) and accidentally discovered that the organizer was a sapient shapeshifting slime... Due to the conservation of mass, her current form was more or less a human-shaped balloon maintained rigid by internal pressure.
After waking up, I though about pneumatic-actuated soft robots and decided to use a hollow body to store the compressed air needed for operation. My personal preference is for internal actuators, as this gives a neater appearance and uses the rigidity of the exoskeleton-like hollow limbs, even if they are less scalable.