You need a unit rate to solve this. A unit rate is "How much does it cost to make 1 or just a unit of the product?"
So, to do that, you jut divide the amount of servings by quarts of soup. Let's do it:
Restaurant A:

Restaurant B:

This clearly means that Restaurant B has a larger serving/quart because it uses less quarts per serving of soup. Hope this makes sense. If it doesn't, feel free to tell me!