him.

Macbeth is forced to rely on mercenaries because his subjects hate him and keep deserting his army. He has shown himself to be a tyrant, caring only for his own power and gain, and they don't trust him. He has the crown but not the respect that goes with it.

Malcolm, on the other hand, is a virtuous and "meek" (compassionate, not bloodthirsty) ruler, like his father, Duncan, was. His troops are with him because they feel a personal loyalty to him and would like for him to become king. They are following him not primarily for money but because they want to help him unseat a terrible tyrant. This makes them better fighters from the start and increases Malcolm's chances of winning battles and the crown.

Macbeth's army acts more out of fear than loyalty. Macbeth has become so evil that nobody would stay out of love for him, or even loyalty, because he is a terrible ruler. Malcolm's army, on the other hand, believe he is the rightful heir to the throne and would give their lives to see him made king. There is no stopping Malcolm's army; Macbeth's, on the other hand, would just as soon abandon him as stay.

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.

Macbeth's army consists of soldiers who stand in "forced affection," meaning they do not support Macbeth willingly or serve him out of respect or loyalty. In Act V, Angus describes Macbeth's army:

Those he commands move only in command,

Nothing in love.

In contrast, Malcolm's army is composed of men who fight passionately, united in their determination to destroy Macbeth, the tyrant who has stolen the throne of Scotland and plunged the country into misery. Furthermore, English soldiers have joined Malcolm in his fight, and citizens of Scotland have risen to swell the ranks of his army, as well. Macbeth notes that Malcolm's army is reinforced with "those that should be ours." The disloyalty of Macbeth's army is pointed out by Malcolm during the battle:

We have met with foes

That strike beside us.

Macbeth's army appears to be fighting for Macbeth, but in battle they deliberately miss when striking Malcolm's soldiers.

If we have a look at Act V of this brilliant play, we can see that the main difference between the two armies is that whatever forces remain with Macbeth are vastly outnumbered by Malcolm's forces. Let us remember that Act V scene 2 features the Scottish lords deserting Macbeth and joining Malcolm's army to oppose Macbeth "To give obedience where 'tis truly ow'd." This scene closes as these Scottish lords move towards Birnam to join Malcom's forces.

Act V scene 3 opens with the news that the numbers of Malcolm's forces now have reached ten thousand, which clearly threatens the forces of Macbeth that do not number this many. Another massive difference is revealed to us in Act V scene 5, when Macbeth gives his troops hope because of the strength of the castle that Malcolm must besiege. Note what he says:

Our castle's strength

Will laugh a siege to scorn: here let them lie,

Till famine and the ague eat them up.

Having to successfully penetrate the defences of the castle is something that gives Macbeth hope at this stage in the play. Of course, as we know, the few soldiers that still remain with him quickly desert him and he is left almost alone to face the invading army. Thus the principal difference between the two armies is based on numbers.