You will often see people asking the question, "Why are there smaller spiders in my enclosure when there was one to begin with?"
There are a few simple explanations....
1. Your spider most likely molted, meaning that it basically shed its old exoskeleton much like a reptile or any other herp would, so it gives the illusion that there is a smaller spider inside the enclosure.
2. Is it a wild caught female? If it is, it might have had spiderlings, but this is highly unlikely because you would've noticed that she had eggs or that there was a web casing inside of the enclosure for the eggs, so most likely your spider has molted.
I hope this helps!
Also a friendly reminder, please DO NOT leave two spiders in the same enclosure together for a prolonged amount of time due to cannibalism.