Quick Answer
Milk does not use the same grams-to-cups value as every other ingredient. Using a practical US cup reference, 244 grams of milk is about 1 cup.
That means 100 grams of milk is about 0.41 cups, while 200 grams is about 0.82 cups.
Milk is close to water in density, but not identical, so recipe conversions should still use a milk-specific reference.