Why do dried fruits swell when cooked?

Dried fruits literally become smaller and firmer when compared to their fresh counterparts.  This is because dried fruits have basically lost much of their water content.  As much as 80 percent of the water is lost when fruits are dried.  Water is the reason that fresh fruits are softer and literally juicier.  Removing the water, though, will also cause the opposite effect and make the fruits thinner, smaller, and water-free.  When the time comes that these dried fruits are used for cooking, they are usually mixed with some form of liquid.  It could be plain water or the liquid sauce from a certain dish, for example.  When there is water involved, the dried fruits basically will rehydrate with some of the water they lost during drying.  When this happens, the dried fruits will expand and swell.  The presence of water alone basically makes the dried fruits expand into a bigger size than it was originally.

The cooking process also involves heat in the process.  While the dried fruit expands as it re-absorbs some water, the heat in the cooking pan, for example, also helps to accelerate the re-absorption process.  The hotter the water or sauce in the dish, the faster the absorption of water by the dried fruits will be.  With this condition, dried fruits will swell to a larger size than before.  Sometimes part of the dried fruit will absorb too much water and somewhat burst during the cooking process.

Fruits are often dried to preserve them so they could be used and consumed for a longer period of time.  When the time comes that people want to use them again for mixing them into some dish or meal preparation, these dried fruits will then re-absorb the water they lost and will eventually swell.  The water used in cooking basically makes the dried fruit swell and become bigger and softer.