What Katan will become 1,000?
Radak: This refers to a small Shevet or family. It will be 1000 times more than it is now.
Why does it say also "ha'Tza'ir l'Goy Atzum"?
Radak: The matter is repeated in different words.
Malbim: This is qualitative increase. "A Katan will become 1,000" is quantitative increase.
Why does it say "Ani Hashem"?
Radak: This good that I promise them, I, Hashem will do it - I have the ability.
"B'Itah" connotes that Mashi'ach will come in the proper time. "Achishenu" connotes that Hashem will bring him early!
Rashi, citing Sanhedrin 98a: If we merit, He will bring him early. If not, he will come in the proper time.
Radak: When the time comes, I will hasten it, to finish it quickly (Malbim - even though now, I am delaying). The salvation will not extend a long time from its beginning until its end.
