Heavy weight does not mean a big rumen on a doe. It means a heavy doe all over.
I participated in ABGA Judges training class in the late 1990's that was taught by South African breeders that came to the US to judge the National show for ABGA. One of the slides in the class showed a big bull. The next slide showed the dam to the bull was not a big cow. They said you don’t have to have big Does to produce large bucks and wethers for goat meat.
The SA breeders said they had been improving on the Boer genetics for over 50 years and they would be working for many more years to continue the improvements. I asked if the new improvements would be larger animals. He said if you want to raise larger animals, raise cattle.
The SA breeders have always been focused on producing better genetics that would produce more goat meat faster, over a longer time and with minimum assistance from the farmer. The Boer associations in the US have always been focused on producing bigger, longer, wider, heavier animals that will win in shows and get Ennoblement labels in the pedigrees.
There are extensive research articles that state excessive weight on Does will likely cause problems in successfully breeding, cause trouble in kidding, negatively impact the health of the Doe with all of the fat around their organs.
This is also one of the potential causes of a goat going into Pregnancy Toxemia. With all of the body fat and the additional weight of the unborn kids, there is less room to hold the feed needed to provide the proper nutrition for the Doe plus the kids.
Just as it is more difficult for a person that is overweight to be able to walk any distance, it is the same for goats. Just as it is harder for overweight people to breath during any minimum exercise, the same is true for goats.
Let us take two different Does, one weighing 150 and the other weighing 250. If you look at the feeding recommendations listed on general goat feed, it says feed around 3% of the animal’s body weight just to maintain the current weight. That would be 4.5 lbs of feed for the Doe weighing 150 and 7.5 lbs of feed for the 250 lb. Doe. That means a breeder is having to feed and extra 1,100 lbs of feed (an extra 22 bags of feed per Doe) over the period of a year just to maintain the 250 lb Doe compared to the 150 lb Doe.
The 250 lb Doe likely will not have bigger kids and may have fewer, smaller kids at birth and have more problems in the breeding, kidding and other health issues. It is not worth it and we don’t focus on breeding for bigger Does.