I will second everyone else and just go down to the two meals a day. You can give her a mid day snack like a few cookies. But her poos can be soft because she is eating to much food and causing the slight weight loss because it is just coming out LOL
Also like most, my adult only eats 2/3 of a cup a day. With the amount on the bags they are just "recommend" amounts. You have to adjust to what is best for your pup/dog and feed them what works to keep them at an ideal weight.