Scorpions need to molt so they can grow. At the end of every instar (the time between molts) the scorpions undergo a certain process called molting. They shed there old skin (called exuvia) and underneath the old skin is a shiny new scorpion. Larger then before and new and improved.
But the process of molting is risky as the process can go wrong. A scorpion can get stuck in it's old skin, or i.e. the metasoma does not come completely come out of the skin. Scorpions can die because of bad molts, therefore it is important to keep captive conditions in the ideal spectrum (See Bad Molts in "Illnesses". Although a bad molt is not really a sickness I found it an appropriate place to discuss this subject).
When scorpions are born, they are instar 1. After they molt for the first time they are instar 2 etc etc. In general, Buthids do not have a fixed number of molts to undergo, before they are adult. Size variations are not uncommon in Buthids and there are large and small specimens, most specimens are somewhere in the middle of their sizespectrum. When scorpions reach adulthood (=sexual maturity), they will not molt anymore.
Here are some pictures of molted scorpions.
Pic 2. Parabuthus liosoma