You are looking at two photos of the same spider, taken two days apart. The spider molted in the interim. Like all arthropods, spiders shed their exoskeleton perodically as they grow. This is called molting. It's amazing that a spider will shed a significant amount of mass and come out seemingly bigger. In this case the spiders front legs came out 60% longer after the molt.

This is a crab spider of species Mecaphesa dubia. I've been raising a batch of them from an egg sac. The male spiders have very long legs, but they only get them after their final molt. I decided to photograph a male before and after its final molt and to include a ruler in each photo. I scaled the two photos so that the rules on the rulers lined up, and then copied the spiders out onto the same image, which you see above. The two images of this spider are to-scale.

It's hard for me to imagine how a spider can pull its legs out of a molt and end up with legs so much longer. Anybody know how this happens?