You're assuming that a monkey that eats 20% less than a monkey that's given the choice to eat as much as it wants necessarily isn't underfed? Maybe. But without some other research that I've missed, that really begs the question.

I think anyone living in a western country has enough data to realize that people eating a western diet tend to be overfed, so the comparison between westerners and Tibetan monks has little to do with monkeys.

In the caloric restriction experiments, the nutritional density of the food the animals are fed is almost always increased. Fewer calories, same levels of protein, nutrients, etc. It's not just a matter of 'less'.

From what I've read, humans on caloric restriction diets have to design their diets carefully, to increase the nutritional density of their own food supply. (E.g. http://www.calorierestriction.org/Getting_started)