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.