Because the earth had lower gravity when dinosaurs weren't extinct?
Elephants live on the low gravity savana?
I think you might find this interesting, as it describes the effects of the grafity on the height:
http://www.livescience.com/33082-would-humans-born-on-mars-grow-taller-than-earthlings-.html
According to NASA, most astronauts grow about 2 inches while they're in space because the reduced gravity causes the fluid between vertebrae to expand. They lose the height within 10 days of returning to Earth's crushing gravity. Because of the growth, NASA uses space suits that have extra room to accommodate the additional height.
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