Insects have a hard exoskeleton, which protects all the other internal organs of the insect. The exoskeleton’s role in insects is also similar to the role of the internal skeleton or endoskeleton that larger organisms have.
As insects grow larger during their lifespan, the size is restricted by the exoskeleton. To overcome this they produce a new soft layer beneath the exoskeleton they have. The outer hard layer then splits and is abandoned. The newly formed soft layer surrounding the larger body hardens and becomes a new exoskeleton. This process of growing larger requires insects to undergo a tedious process that is difficult to repeat regularly. Therefore insects do not molt more than 3-4 times during their lifespan.
The presence of the exoskeleton is one of the primary reasons why insects cannot grow very large in size.