Context: The most obvious difference between the brains of mammals and other vertebrates is in terms of size. On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as large as that of a reptile of the same body size.

Question: A mammal's brain is how many times larger than a birds relative to body size?
Answer: twice as large

Question: A mammal's brain is how many times larger than a reptiles relative to body size?
Answer: ten times

Question: The biggest difference between brains of mammals and other vertebrates is what?
Answer: size.