Like all other plush toys, the inside of the plush balls is also filled with soft synthetic fibres. Rather than using just one type of stuffing material, plush toy manufacturers often use a mixture of things like batting, straw, cotton, etc. The quality and softness of plush toys vary depending on the quality of the plush inside the toy. When making large plush toys like teddy bears, they try to make it bottom-heavy to keep the animal stay upright when kids are playing with it or hugging. In order to do that, they use heavier plush at the bottom and lighter plush at the top. But for balls, it will be a homogenous mixture.