Well when the bags say 'Cups' they actually mean measured cups as in 8 Ounce - as long as your 'mug' is 8 ounces, you are fine but 6 cups of food aday seems like alot to me !
Different food brands also call for different quantities. Low grade food, actually has you feeding MORE, so its not really as big of a bargain as people think.
Hershey is 10 months old now, and is fed 1 1/4 cups at Breakfast and 1 1/4 cups at Dinner for a total of 2.5 cups aday
Gizmo is almost 3 years old, and is fed 2-2.5 cups at dinner.
1 Cup or 8 Fluid ounces equal about 240ML if that helps you