Principles to help you figure it out: Referent and Compared Quantity are all about the role they play in the sentence--it's a sentence structure thing. The word "than" points to the referent.
1. Sarah has 3 fewer beanie babies than Kylie.
2. There are 5 more people wearing red T-shirts than blue T-shirts.
3. Sam has 2 more large teddy bears than small ones.
4. There are 4 fewer erasers than pencils in the drawer.
5. Sally is 3 inches taller than Marie.
6. A batch of pancakes uses 2 cups less sugar than a batch of cookies.