This may be a longshot but by any chance do you feed her the high price petfood you buy at the vet..
Years back I fed my dog the science diet food that you buy at the vet. They were to eat less as it was more filling (alot of food on the shelves at stores are just filler foods) . SO because its more filllling you have less stool.
This was the idea for this food too I was told at the time.
MY dog was put on this because she was allergic to eveything.