One individual doing one Mitzvah can save the entire world. If an individual wholeheartedly repents, the previously balanced scales will most certainly tip in the direction of merit, both for the individual and the entire world. Any and every individual has the power literally to save the world. Repentance is the root of that power. The Anaf Yosef explains this power of repentance in another fashion.
Repentance has several components.
When a person fasts and weeps as part of the repentance process, he has removed G-d's wrath from the world. Rather, He judges with compassion and mercy. When G-d judges with mercy, He accepts repentance that is not complete. He accepts from many only a confession without the fasting and weeping that should accompany it. The person who sincerely repents causes the entire world to be viewed in a more favorable light, enabling many more to have their less than wholehearted repentance accepted.