Question about Breeding Guinea pigs?

Hello i am nick and i am going to breed to guinea pigs nothing you can say will stop me as i have been planning this for a year and am confident that nothing will go wrong but my question is my guinea pig is a little on the small size and needs to gain 3 ounces by may how can i do this . So any advice on getting my Guinea pig larger without making her fat would bre great thnx .