Macbeth's army is largely made up of mercenaries: soldiers who hire themselves to whoever will pay them the most. They have no direct sense of loyalty to Macbeth, and they would probably desert him at the drop of the hat. They can't be expected to fight valiantly for 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.