Question

why were the first animals tiny and soft bodied

Answer 1

Soft-bodied organisms are animals that lack skeletons, a group roughly corresponding to the group Vermes as proposed by Carl von Linné. All animals have muscles but, since muscles can only pull, never push, a number of animals have developed hard parts that the muscles can pull on, commonly called skeletons.

Answer 2

@naresa That does not answer the question and you are looking at animals as if the term is nearly equal to mammals, which it is not.

@nicolepuerto The first animals are thought to have evolved in marine environments, and would not have needed any sort of a skeletal system to support body weight and allow for movement. It is also thought that the first animals is where multicellularity evolved in the phylogeny of animals and had very simple body plans, perhaps without true tissues. This means that the first animals also lacked a vascular system. If there is no vascular system, then there is a physical constraint on body size because all nutrients and chemicals required for life must be distributed via diffusion/osmosis. This is why there are no 10 feet tall mushrooms or moss (note, neither are animals).

So, the first animals were small because they could not grow larger and survive without a vascular system, and were soft bodied because a skeletal system, even a cartilaginous one, was not required for survival in a marine environment.