From information gathered from various places (including this site I'm sure), females can take up to 3 years to be ready to breed. I would focus more on keeping them healthy than getting them up to breeding weight right now. Feeding more than what they really need or want would be called power feeding and most people would frown upon that practice because it can cause issues for the animal and a shorter life span. Getting bigger faster doesn't always mean healthier. As long as they are healthy and eating well, I wouldn't try to push their weight at this point. They're still pretty young.