Yea cause i totally "let" her get pregnant. Were not all of the upper crust and able to control every dimension of our dogs and our own lives.
You know there's a surgery to prevent pregnancy, right?
I find it surprising that you work at a shelter, yet you let (yes, YOU are responsible) your mixed-breed dog get pregnant. Perhaps you have not worked at the shelter long enough to have seen whole litters of puppies and kittens get euthanized.
What does the "upper crust" have to do with anything? Are you saying you couldn't afford to get her spayed? I dearly hope she is spayed now.
As for her size, yes, you probably stunted her growth by allowing her to have a litter so young. Then again, you really don't know how big she was going to get. She could be a GSD/Cocker mix for all anyone knows and might have only been 40 pounds anyway.
Once a dog has reached a certain age, you cannot compensate for stunted growth. Feeding her more will only make her fat, which you definitely don't want. If you simply want to take better care of your dog, you can start by feeding something better than Pedigree and table scraps.