I don't see why she wouldn't gain weight while in heat. Don't us women gain some weight that time too? It is probably water weight gain like the rest of us get. It is a good thing for Chubbyness to occur. It means she will probably breed better and have an easier birth. If your alot concern then reduce her meals and cut out some treats. She should slim down after her heat is over. I had a Golden Retriever that did the same thing.