I agree with bmadnick--the key difference between the two armies is in the degree of love and loyalty the soldiers feel toward their leaders. Those who fight with Macduff fight for their country and for their rightful king (Malcolm, not Macbeth). Those who fight with Macbeth fight because they are forced to. Macbeth uses intimidation and fear to keep his people under control. This is important because MacDuff's army will fight to the death if they have to to keep their country and their families safe while Macbeth's army just need an opening to leave. Macbeth's army feels neither love nor loyalty for him. Thus, although he appears strong, he is weak for when the conflict comes his army will easily break.
While it is true that Macduff’s army has right on its side in fighting the tyrant Macbeth, that “right” is very personal. Macduff’s determination to overcome Macbeth is motivated in part by his desire to vindicate himself (he originally left Scotland because he feared he would be held accountable for Duncan’s murder), and in part by his desire to seek revenge for the death of his family, murdered by Macbeth’s men. When confronting Macbeth in 5. 8, Macduff says to Macbeth, “Tyrant, show thy face! If thou be’st slain and with no stsroke of mine, / My wife and children’s ghosts will haunt me still” (14-16). It is also worth noting that Macduff’s courage grows when he learns that because he “was from his mother’s womb / Untimely ripped” (5.8.15-16) he will be able to kill Macbeth where others could not. Macduff also shows his complexity—and fuller manhood--when he finds he is unable to seek this revenge until he properly grieves the loss of his family, and this affects his leadership of his army. When Malcolm tells Macduff to “dispute it [the murder of his family by Macbeth] like a man,” Macduff, replies “I shall do so; / But I must also feel it as a man” (4.3.219-21). Unable to feel grief, Macbeth feels guilt, and defines his manhood through his acts of violence, which affects his leadership
Macduff's army is fighting to save Scotland, so their hearts are in their fighting. They will fight harder for their country because it's something they believe in. Macbeth's army is fighting out of fear of Macbeth. They really don't want to be in the fight because they don't want to die. They don't have a "cause" to believe in. People always fight harder when they believe in what they're fighting for, and it's more difficult to defeat them.
Macduff leaves Scotland to join Malcolm's army in England. Malcolm fled earlier when his father, King Duncan, was killed, because he was afraid that he would be next. Once Macduff realizes how dangerous Macbeth is, he goes to England to get help from Malcolm. They return with 10,000 men and fight Macbeth's army-- fulfilling all of the witches' prophecies.