Because they contain less "built-in" water.
Explanation:
When two glucose molecules 'decide' to bond, they lose a molecule of water:
<span>2<span>C6</span><span>H12</span><span>O6</span>→<span>C12</span><span>H22</span><span>O11</span>+<span>H2</span>O</span>
And this happens at every connection between glucose molecules, so in a long chain a lot of water is lost. The water does not contribute anything to the energy content of the compound.
External stimulus
The stimulus to which the organism is adapting to is outside of the organism's body.
