How did you move his weight down from 43 to 39? My girl was 47# when we got her one year ago at 6 yrs old. I feed her 1/3cup 2x daily. I make her treats myself. Very low cal. She has very minimal exercise. We walk everyday, but she walks very slowly. Her weight is not budging. She is a very large Corgi like you describe yours.
Oh bless her, she's a chub! I know it is NOT because of you at all but she needs to lose 10 pounds at least.
What is the calorie count on the food? We feed one that is only about 300 calories a cup. I've seen foods as high as 500, which would make a huge difference. Make sure the primary protein source is meat and not corn gluten or soy protein. Adding in treats and food, my dogs probably average something around 400 calories per day.
I think adding exercise is the key. You need to get her moving. Think of breaking up the day with repeated small bursts of exercise to get her metabolism moving. So for instance, when we come home from work we have a game of chase. While we're watching tv we throw a ball. Considering her weight you don't want her working too hard for too long, but try to get her moving as often as you can and build up from there.
Have you had her thyroid checked? That could cause issues if it's low. If that's ok, you may need to cut her back even further, to a quarter cup 2x a day. Check with the vet; you may need a supplement for vitamins and minerals if you are feeding that little amount.
Please keep us posted on how your girl is doing!
I should add Jack now goes about 35 pounds but he is a RAIL. I keep him super thin because he goes banging around the house, chasing tennis balls and skidding on the tile and slamming into the wall. He's back up to 1/2 cup 2x a day and maintains weight at that; sometimes we need to give him extra. Once he lost the weight, we had to up his food and he is back to what he got fat on. Not sure how that happened.
Our Maddie gets 1/3 cup AM and 1/2 cup PM but sometimes she puts on a pound or so and we cut her back for a week or so. I don't keep her as thin as Jack because she's easier on herself. She usually goes about 28 pounds and is heavier proportionally than Jack at 35.