At many times in the past there have been lots of large animals. The present is rather the exception, probably because of a one-two punch from the ice ages and human hunters. Largeness is itself a specialized adaptation.
True, but that does not point to gigantism being favored. Certain lineages may have larger-than-average members, but the overall average itself remains remarkably stable through time. Similarly, many lineages have smaller-than-average members, but small size, like large size, represents, as you say, specialized adaptations, not expected trends. As such, there is no a priori reason to assume that any given lineage will, over time, grow larger than the average.
My suspicion is that the show’s producers are operating from the assumption that as there were a number of large mammals (and larger-than-average birds and reptiles before them) before man, once we are gone things will revert to this state. Either that, or they are simply following Olentzero’s marketing scheme: bigger critters = more sensational = more viewers. Seeing as how it’s all speculation anyway.
Very true; I guess what I was trying to say was that given an open econiche for large animals (a stable environment and a lack of human hunters) adaptive radiation would eventually produce those large animals. Gigantism is a sort of “economy of scale” thing: it’s a winning strategy if conditions are right. Our current era is unusually unfavorable to large megafauna; about like trying to run a Walmart in Afghanistan.