Here is how volume/weight conversions work. A gram is a unit of weight. A cup is a unit of volume. Volume is the amount of space an ingredient occupies and weight is the mass or "heaviness" of an ingredient.
500 grams of mini marshmallows = 10 cups
500 grams of flour = 4 cups
500 grams of sugar = 2.5 cups
As you can see, the weight for these ingredients are the same, yet the volumes are different. This is because each ingredient has a different density.
Here is another example:
A cup of feathers and a cup of lead pellets both occupy 8 fluid ounces of volume. The feathers will weigh a lot less than 8 ounces on a scale and the lead will weigh a lot more than 8 ounces on a scale.
Asking how many cups equal 100 grams would be the same as asking how many hours are in a 100 inches. Volume and weight are two completely different measurement systems... just as time and length are two different measurement systems. The only difference is that volume and weight can be linked through density.... mathematically speaking. Density = Mass divided by Volume. Density can change for the same ingredient. For example, a cup of sifted flour will weigh a lot less than a cup of unsifted flour. A pint of water will vary in weight depending on the temperature of the water.
There are websites that do volume/weight conversions based on density. Two great websites are:
or, you can look in a cookbook that has a volume/weight equivalency chart.