It took 24 million generations for mammals to evolve from the size of a mouse to that of an elephant, a study has shown.
Shrinking is a much faster process, however. Large-scale reductions in size leading to dwarfism only take around 100,000 generations.
Scientists writing in the journal Proceedings of the National Academy of Sciences looked at 28 groups of mammals, including elephants, primates and whales.
Size change since the age of the dinosaurs 70 million years ago was tracked by generations rather than years to account for varying life spans between species.
"We can show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change - a massive change, but also a very long time," said study leader Dr Alistair Evans, from Monash University in Melbourne, Australia.
Changes in whale size occurred at twice the rate of land mammals, the scientists found.
"This is probably because it's easier to be big in water," said co-author Dr Erich Fitzgerald, from Australia's Museum Victoria.
Decreases in body size happened more than 10 times faster than increases.
Living on small islands with limited resources is known to promote evolutionary dwarfism.
"When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands," said Dr Evans.