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> By carefully measuring food, one loses and gains weight accordingly (taking into account age, physical activity, etc.)

That was the problem. The lab animals have had their food careful measured for several decades with essentially the same food/lifestyle, but weight was being gained at a rate higher than other animal populations. Because it's unknown the "why" part is speculative, but they've got some interesting ideas. Changes in animal husbandry standards, endocrine-disrupting chemicals and infectious agents were considered as possible factors.

From the study:

"There are multiple conceivable explanations for these observations. Feral rats could be increasing in weight because of selective predation on smaller animals or because just as human real wealth and food consumption have increased in the United States, rats which presumably largely feed on our refuse, may also be essentially richer. But these factors cannot account for the findings in the laboratory animals that are on highly controlled diets, which have varied minimally over the last several decades." (https://royalsocietypublishing.org/doi/full/10.1098/rspb.201...)