That doesn't cover lab animals. I've also read that animals in the wild also weigh more. One explanation is higher global CO_2 levels, giving faster growth, wh

That doesn't cover lab animals. I've also read that animals in the wild also weigh more. One explanation is higher global CO_2 levels, giving faster growth, which means more calories (and fewer other components). So, the same amounts of food now have more calories. There's also arguments for hormones in the general food supply. Possible global factors are pollution in general, and plastics in particular. (For wild animals, decrease in biodiversity may be a factor).