Hi there,
I can understand why you are worried about Sasha, but I wouldn't stress too much yet as she sounds happy and healthy in every other way
Most animals scoff down their food but some can be very picky. You may need to try different foods to see what she likes and maybe use a variety to stop her getting bored. Just beware of changing any food too quickly as this may cause diarrhea.
Dogs are like people in that some are thinner than others. Weight is a good measure but I prefer body condition score. Look at Sasha from above - she should have a waist (ie from her ribs to her hips she should come in a bit), and when you look from the side her stomach should curve up a bit too. Her ribs shouldn't be really easy to see but should be quite easy to feel.
Unfortunately I think we get used to seeing overweight dogs, so thinner and probably healthier dogs look underweight.
Depending on the food, 2 cups might be too much for Sasha. As long as she is still running around and being active I wouldn't be too concerned. If she becomes lethargic or is really not interested in any food then a trip to the vet may be necessary.
Let us know how things go
Kate