Cope's Rule.
Edward de Cope - American Paleontologist - argued that organisms have a tendency towards increase in body size during evolution.
although there are many exceptions, the rule is shown by evolution of mammals like horse and camel exhibit increase in size.
MJ Benton 2005 explained factors behind cope's Rule
Larger organisms find it easier to avoid or fight off predators and capture prey, to reproduce, to kill competitors, to survive temporary lean times, and to resist rapid climatic changes.
benefit from better thermal efficiency, increased intelligence, and a longer lifespan.
Limits
Larger organisms require more food and water.
Longer period of reliance on the mother.
Restricts the clade's ability to evolve rapidly in response to changing environments.
There are probably also physically imposed limits to the size of some organisms; for instance, insects must be small enough for oxygen to diffuse to all parts of their bodies, flying birds must be light enough to fly,
An extensive study published in 2015 supports the presence of a trend toward larger body size in marine animals during the Phanerozoic. N Heim etal. 2015.