This article is not stating that lab rats are overweight. It is stating that some rats were made overweight by dosing them with these chemicals.
There was a 2010 study that did suggest lab animals were fatter than previous years. However, it was not statistically significant for rats. Only mice. It did not specify a causal factor.
This article is."A significant rise in obesity incidence has even been noted in laboratory rodents and primates – animals raised under strictly controlled conditions of caloric intake and exercise. The only possible factors driving weight gain for these animals, researchers believe, would be subtle chemical changes in the nature of the foods they eat, or in the materials used to build their pens."
