Oh, good. I guess you learned the hard way that you can never leave unaltered males and females together, even for a second and even if you are supervising. I only hope your girls won't have to pay the price!
It sounds like you keep your guinea pigs outside. It is much safer for them, and nicer for them and you as well, to keep them in the house with you. Have you ever looked at the cages at
Guinea Pig Cages, Your Cavy At Home ?
I wouldn't worry about your guinea pigs being overweight, unless they are showing other abnormal signs--are they sluggish?