1)
What is "ha'Yotzeis Elef"?
1.
Rashi: A thousand men come from it.
2)
Why will only 100 remain?
1.
Radak: All will die via the sword, hunger and plague, and the rest will be exiled. "Ha'Yotzeis" is not Po'el Yotzei (it acts on others), rather, Po'el Omed (the action applies only to the subject). Matters of people of the city are attributed to the city, e.g. "va'Tavo ha'Ir ba'Matzor" (Melachim II, 24:10), "v'Im Lo Sashlim Imecha v'Asesah Imecha Milchamah" (Devarim 20:12). Since 100 remain, it is as if the city leaves them over.
2.
Malbim: This shows that "Lo Sosif Kum" (2) means by herself. Since few remain, via nature she cannot strengthen herself.
