My answer would be that because they start to take up a liquid-like form, and the energy-minimum is achieved when they have the smallest surface, like bubbles, or in space water forms spheres. When you heat it, it starts to melt, and tries to have minimal surface, so it contracts to achieve that...
If you start to melt an iron cube in a spaceship, you would see, that it becomes a sphere. On the earth gravity does not allow liquids to form.
In physics things grow from heat because the molecules resonate faster, taking up more space around them, but with rubber, the same happens but it starts to melt before reaching a significant volume-change.
Anyway this is how I see it.
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