[Lord Halsar]

We must remember one thing when it comes to actual strength. Melkor was diminished in power by spreading his own strength through his servants (and the world), and I would guess that he did not regain this power once they died. And to this I also say, Sauron was no fool. He would not make the same mistake Melkor did in powering his troops with his own being. So, while Sauron churned out hundreds of thousands of soldiers for his own uses, Melkor, even with his imprisonment, would still have all of those bits of strength continue to draw away from him. If indeed he was not getting this power back, he would have two Ages worth of lost power on (or perhaps off) his hands.