It is true that Melkor was the mightiest, "Ilúvatar spoke...Mighty are the Ainur, and mightiest among them is Melkor". [Sil., Ainulindalë, p. 6] This is mentioned many times throughout the Silmarillion.
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The mightiest of those Ainur who came into the world was in his beginning Melkor" [Valaquenta, p. 18]
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It is also recalled that, "so great was the power of his uprising that in ages forgotten he contended with Manwë and all the Valar" [pp. 25-26].
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The greatest power under Eru (sc. the greatest created power)... in the early age of Arda he was alone able to drive the Valar out of Middle-earth into retreat. [MR. p. 390]
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However, Melkor in his beginning is not the same as the Morgoth in the North of Beleriand. It is said in Morgoth's Ring, "Melkor had already progressed some way towards becoming 'the Morgoth, a tyrant (or central tyranny and will), + his agents'. Only the total contained the old power of the complete Melkor; so that if 'the Morgoth' could be reached or temporarily separated from his agents he was much more nearly controllable and on a power level with the Valar." [p. 390]
His power was at this point dispersed, so that instead of being concentrated in himself alone his servants had a share in it to the point where only with himself and his agents all-together could he come close to the power he once wielded. When he and Manwë come face to face we are shown how they both note the difference in Melkor, "Both are amazed: Manwë to perceive the decrease in Melkor as a person; Melkor to perceive this also from his own point of view: he has now less personal force than Manwë, and can no longer daunt him with his gaze.... he is 'dispersed'." As you can see he was shocked to learn of the diminishment of himself as well. Perhaps among his servants he did not recognize it because compared to any one of them singly he was so much more powerful. But when he comes face to face with his peers and sees that he is lesser than Manwë this is crazy news. His agents the Valar could easily deal with and Morgoth now was more like to the Valar in power when they separated him from them.