Can you observe some of the animals around you? It would be interesting note that most big animals such as a cow, a dog or a cat have strong internal skeletal structure. Now look at some small organisms such as a butterfly, a beetle or an earthworm. They do not possess an internal bone system. Does size have anything to do with such differences?
Most invertebrates are small because they lack internal skeletons. An external skeleton is an inhibitor and makes it difficult for an animal to grow further. If it grows too big, it will not fit inside the skeleton. If the skeleton grew bigger to accommodate the animal’s increasing size, it would become too heavy for the animal to carry.
However, it would be wrong to assume that all invertebrates are small. Invertebrates that live in the sea can grow quite big. There are giant clams in the South Pacific that are 1.2 metres across, and weigh a quarter of a tonne! Giant squids can reach a length of 17 metres, and weigh a tonne!
Picture credit: google