As with all Arthropods, the body of jumping spiders is covered and protected by an exoskeleton. Normal growth therefor is almost impossible. They need to mold to grow bigger.
Once the old suit is getting to tight, the shield of the behind body unfolds and the spider is rising the pressure of the body fluid (hemolymph) to pump up its front body until the body shield breaks open. Afterwards the behind body is freed and the legs are being pulled out of their shield under powerful exertion. The whole process takes several minutes to complete.
The now fresh spider does some gymnastics to keep the joints flexibel during hardening of the new tuxedo. This whole process is repeated five times until maturity is reached. The intervals between molding are very short in the nymph state and are getting longer with age, depending on the food supply.
The pictures show the difference in size between a juvenile and a mature animal. Both pictures where captured in the same scale of approximately 3:1 on an APS-C sensor.