Growth via molting generally works like this:

1. Inside its exoskeleton, the arthropod grows.
2. When the exoskeleton finally gets too tight, the animal begins the molting process by puffing itself up and splitting the exoskeleton.
3. The new exoskeleton is still soft, so the arthropod wriggles out of the old armor very carefully to avoid damaging its still-vulnerable body.
4. Then, the arthropod waits — sometimes for days — without protection or body support for its new armor to harden before life as usual begins again.

These images, in sequence from 1 to 5, show a blue crab molting. The crab emerges from its old exoskeleton significantly larger than it had been.

Crab molting photos provided by the Smithsonian Environmental Research Center and Alicia Young-Williams