There are two competing models.
Vertebrates (I.e. fish, amphibians, reptiles, birds, mammals) have their hard, structural components on the inside of their bodies (bones). This means that their exterior is softer and more prone to damage, but it is also easier to grow larger when abundant food sources are available. Their bodies can also expand and contract, which means that they can draw large amounts of oxygen directly into their bodies through breathing.
Invertebrates (I.e. crustaceans, insects, arachnids) have their hard, structural components on the outside of their bodies (exoskeletons). This means that their exterior is hard and impervious to damage, but they are also fundamentally constrained by it. To grow, many must go through phases of metamorphosis where their bodies are effectively dissolved and remade, and then in adulthood must periodically shed their exoskeleton in order to grow incrementally larger. They cannot have lungs as their bodies are rigid and unable to contract; instead they must have tiny pore-like holes in their exoskeletons down into their bodies, called spiracles, through which oxygen can be exchanged. It is in particular this second factor that prevents invertebrates from growing to the size of vertebrates, unless the oxygen content of the atmosphere is drastically high (as it has been in the past). Also, more body types and shapes are available to invertebrates.