Could be because she wants to eat it versus not wanting to eat your previous food. Could also be because the food has more of something than the old one.
i.e. I was feeding TOTW, my dogs were getting chubby off of it. I was feeding them 1c/day like I was with Nature's Variety. They weren't getting fat on NV. I decided to switch to EVO and cut down my older dog to 2/3c day and the younger (who is more active) stayed at 1c/day. But he is getting fat on that, so I have to cut him down to 3/4c.
Different foods have different properties. I was told TOTW has too much carbs in it. Still a good food, but just changing to EVO and cutting back a bit, has helped their weight a lot.
Just my story. When I first transitioned to any no-grain food, they immediately gained a lot of weight. With no-grain and high quality, you don't need to feed them a lot (well, that's my case).
Oh, and when I talked to the worker bee in the pet store, she said to just go by the back of the bag. I would've been giving them 1.25c/day. That's a LOT. But the vendor of the company came by, we talked about the size, weight, shape of my dogs, etc. and he suggested a more correct portion.
