Bigger (slower), smaller (faster)

We may be seeing a human obesity epidemic in some parts of the world but according to a recent study on the evolutionary scale it is much quicker for animal species to decrease in size than grow in size. Apparently it takes about 24 million generations for a mouse sized animal to evolve into the size of an elephant. However a mere 100,000 generations are needed to reverse the process and achieve dwarfism. The differences may be explained by the evolutionary driving forces of smaller species requiring less food and being able to reproduce quicker.