Well... probably depends on the breed.
Any high production milk breed will have obvious hips (both cows and goats ).
It is better to watch the body condition score of the animal.
Here is an excellent video that goes over the body condition score for milk goats:
As long as you watch the feed of the milk goat, a gallon a day of milk production for over a year long lactation is easy to maintain.
Those ladies that produce 2 gallons a day, those are more difficult to keep the weight on!
However...your choice if you wanna drink the milk yourself, let the kids have it...or just let her dry up.