One cup of rolled oats has a lower density than either, weighing 3 1/2 ounces. Two and one-quarter teaspoons of instant dry yeast are equivalent to 1/4 ounce or 7 grams.
The mass of 1 cup any ingredient depends on that ingredient's density. Some people may notice that liquid measuring cups mark 1 cup as equivalent to 8 ounces. Water and other liquids are much more dense than flour, so 1 cup has a much greater mass and weight.Learn more about Food Measurements