The beginning of the words of his mouth is foolishness... Ecclesiastes 12:13
"Evolution tends to make animals larger over time – except when it 
makes them smaller. In Science, Kaustuv Roy lamented the perils of this principle that animals evolve
toward largeness, known as Cope’s rule.
It has some examples but plenty
of exceptions.
If Cope’s rule were a law of nature, wouldn’t we find
lots of giants? “Instead, most species tend to be small to intermediate
in size, with few in the smallest and largest size classes.”
Roy argues:
Translating these “rules” into predictions about trajectories of size evolution is not straightforward. If bigger really is better, then we should have a world full of giants, yet most species are small. Clearly there are costs to getting bigger, which prevent a runaway Cope’s rule. Such costs involve complex interactions among a multitude of factors including development time, population size, and patterns of resource use. In addition, the temperature-size rule [Bergmann’s rule] suggests that the external environment, which changes in a complex and nonlinear manner over geologic time, is also important in driving size evolution. So, not surprisingly, simple process-based models of size evolution (such as one based on energetics) have not been widely accepted.
As if that weren’t complex enough, “There is also the problem of scaling up from observations at the population level to macroevolutionary trends in size,” he said.
It is unclear whether models built on samples from a few generations (living or extinct) will hold up “across geographically separated populations and macroevolutionary time.”
The uncertainty about these “rules” of body size evolution make it
difficult to quantify the apparent influence humans are having on large
animals today." CEH
