Evolution - small to big?

giant ground sloths? Nothrotheriops shastensis was about the size of a brown bear whereas Mylodon darwinii was about the size of an elephant...they lived at the same time...and at the same time there were sloths (also now extinct) that were the same size or smaller than living tree sloths such as Bradypus or Choloepus. Mammoths on the channel islands were dwarfs and a close relative (now extinct) of the Asian elephant was about the size of a very small bear....there is no apparent trend in size increase or decrease during evolution except that often, species on islands tend to be smaller than their mainland relatives...though this is also not absolute...as to a correlation between size and complexity....do you believe then that elephants are the most complex land mammals and whales the most complex animals period?...they are after all the biggest.

Though you did qualify your statement by saying "sometimes" the fact that small and large species of sloths, elephants, etc. lived contemporaneously argues against oxygen as the main factor in determining size. By the same arguement why arent minke whales as large as sperm whales or blue whales? Presumably there are not locallized areas of higher oxygen content exactly where these different species exist since in some cases their ranges overlap.I don't really see a tendency in evolution regarding size...mammoths (my favorites) varied over time in size getting bigger and smaller depending on the time with Mammuthus columbi being absolutely enormous and M. primigenius smaller than modern African elephants.If the argument is that dinosaurs, extinct mammals etc. are big versions of modern taxa as a result of different oxygen levels, then how does one explain that some modern species are larger than their ancestors while others are smaller..i.e. no generalized trend.