The following viewpoint comes from David Hardesty,and it succinctly answers many of these obesity questions:
Some sows, as with some women, are genetically prone to be very "curvaceous". We consciously select for animals which develop the wide shoulders, broad, cobby bodies and high crowns. This gets passed onto their offspring. Those mature, senior cavies with the beautiful high crowns, wide loins and full hindquarters are really full of fat. Sows, which have been set aside by themselves for showing, run the risk of not getting pregnant unless they loose some body fat.
Such an animal may have an extremely difficult time breeding because her ovaries are swaddled in layers of fat, and the internal body heat, retained by the fat deposits will sterilize the female. Another problem, with all this internal fat, is that of the mechanical blockage of the oviducts in the female, by fat deposits, preventing sperm from reaching the ovum, thereby preventing conception. In the male cavy there is a similar problem with obesity. In male mammals, the gonads are carried in the scrotum which keeps the gonads cooler than body temperature. Therefore boars, whose gonads do not drop into the scrotum, will invariably be sterile because the body temperature inside the abdominal cavity is too hot for the production of viable sperm. Fat males sterilize themselves because their body fat constantly covers scrotum and the sperm dies from being surrounded by all the heat trapped by their fat.
Now I hold back breeding obese animals as many do not get pregnant and those that do are not healthy enough (in my estimation) to successfully carry and deliver a litter. I've put them in larger cages with water at one end and food at the other, but my space is limited in my house, so their cages are limited in size to 40" x 24". In the wild, the cavy evolved as a herding family unit which provided a primary line of defense for grazing cavies, against predation. As breeders we build or purchase caging built from welded wire with metal trays in the bottom to hold shavings. These cages are built in dimensions, which are in various multiples of 6", to minimize wastage of the width of the welded wire rolls utilized. We might use a particular style of welded wire cage because either the breeder you purchased stock from used them, or that‚s all you could buy and ship in. If they work for them, it will work for me, you think. But these cages are unnatural, and are, in the long run, potentially harmful for the cavy because they rarely permit enough room for the cavy to run as they would in the wild.
Cavies in a herd, in the wild, get a tremendous amount of exercise from the constant walking and running. This exercise, in addition to low fat diets of grasses, promotes lower body fat percentages, which prevents problems with heart disease, high blood pressure. Cavies suffer from all the same afflictions associated with obesity that humans do. Cavies of good breeding size are not as heavy as a cavy primed for showing. We set aside cavies to prepare them, condition for shows. But what we are really doing is fattening them up! Animals with very high deposits of fat within the abdominal cavity are said to have "excellent flesh condition" because they feel hard as a rock.
Cavies, which have a healthy percentage of body fat, are much better breeders, but tend not show as well. Cavies, which have a healthy percentage of body fat, may be described as "narrow in the shoulder and mid-section." Sows which are just coming off of nursing a large litter will often, despite our very best efforts to keep them well nourished, be very thin. This is because her body fat is leaving as milk fat. Given a couple weeks away from the babies and she will balloon back up to normal.
My animals are constantly provided with grass hay (right now some fresh orchard) and Timothy. I feed Purina G. Pig Pro Lab - lettuce (romaine) twice a week and carrots once a week. We can't make them diet like we do by withholding or rationing their food. One of my breeder friends has two animals that need weight-reduction for better health and appearance AND she wants to enter them at the Nationals.
About 2 months ago I completed the gradual conversion of my entire 300+ herd from Buckeye Nutrition 22% Cavy Chow, over to 18% Manna Pro Family Ration. They get their Vitamin C through the watering system. I have never felt harder flesh condition on my animals. Harder as in not fat. Especially in the younger, growing animals. It's too late for the older animals. The damage has been done. What damage? I hear you ask. Obesity from cavy chow which is much higher in high calorie, starch laden corn! The rabbit pellets look dark green next to the decidedly yellowish looking cavy chow. I've actually had a senior boar, unshowable because of pea eye, whose pea eye has shrunk to almost unnoticeable proportions. Because, I believe, the 18% rabbit feed represented a lower calorie diet that the 22% cavy chow, which coupled with the 8' long tubs I employ enabled him to loose body fat.
Well when I lived in an apartment I had a 4' oval sheep watering tank under the kitchen table. An old trick Chuck Hawley used to employ to get big, fat, show sow to breed was to release her down on the floor with a supply of feed and water. Give her a month running around on the floor (like she would be naturally in the herd) and she will loose weight, perhaps enough fat to get pregnant when put with a boar.