One serving (1 cup) from the fruits group is equal to 1 cup of fruit, 1 cup of 100% fruit juice, or 1/2 cup of dried fruit. Why is the serving size for dried fruit smaller than the serving size for other forms of fruit?

Because the dried fruit contains less water

Step-by-step explanation:

Drying is a phenomenon whereby heat is used to remove moisture from a material.

Fruit juice contains about 85% water, that means the remaining fruit content is 25%

The average water content of fruits is about 89%, this means that the remaining fruit content is 11%.

Dried fruit contains about 20% water which means that the remaining fruit content is 80%.

If we assume that each cup serving weighs 1kg this implies that;

The real fruit content (vitamins, minerals etc) masses are;

Fruit juice = 0.25kg

Fruit = 0.11kg

Dried fruit = 0.8kg

We can see that the mass of the fruit content in the dried fruit is twice more than than of the fruit and fruit juice individually. This is why half cup of dry fruit is equated to one cup of the other two.