Explain why a car with a large mass might use more fuel than a car with a smaller mass?

Because, according Newton law F=ma. If F=ma, then A=F*L=E. (this is physics)

Where F -force

m - mass of car

a - acceleration of car

A - acceleration work

L - trace

E - energy

And so, if mass of car is large, then spent more energy, and spent more fuel.