What would explain the change in the size of a gummy bear after it is placed in regular water?

The change in size of a gummy bear when placed in water is due to the presence of gelatin in the gummy bears.

Explanation:

Most of the gummy candies, like the gummy bears are made up of gelatin. To make the gummy bears, water and gelatin are mixed and heated. Then, this mixture is allowed to cool. During cooling, the water evaporates leaving the gelatin in a chewy form.

When the gummy bear is placed in water, the water is absorbed by the gummy bear due to  osmosis. Hence, the gummy bears tend to increase in size.

Water is a hygroscopic substance i.e it tends to absorb water. So, this property of gelatin will enable it to absorb water and increase in size.

the answer is:

carbon dioxide