Dinosaurs actually came in all sizes and shapes but certainly many were big and some of the sauropods were absolutely huge. One of the largest was Brachiosaurus, which was up top 22 metres long and 14 metres tall. Its weight is disputed, ranging from 35 to 80 tonnes, but either way it was a heavyweight. Recently there have been reports of even bigger sauropods. These are mostly based on rather fragmentary remains, but if scaled up do suggest animals considerably bigger than Brachiosaurus. Ultrasaurus may have been 30m long, Supersaurus up to 40 m long, and Seismosaurus might have been as much as 45m long.

The size of these creatures is probably a simple consequence of how they fed. Vegetation is a tough diet for all animals and sauropods were high browsers relying on the particularly tough leaves of conifers. Their teeth were simple and designed for nipping or raking foliage from trees, rather than for chewing. They had stone laden gizzards which they used to grind up the tough leaves, but breaking down the plant cells could only be achieved by using the stomach as a huge fermentation tank. Sauropods had to be big because they had to contain a huge stomach. And being big is a useful adaptation in itself. It gives some protection against predators - unless they too evolve bigger. It also gives more insulation, either to prevent overheating in the sun, or to avoid heat loss when it gets cold.