There is an example of gas exchange in the lungs that is appropriate for this as well.

Think of the available space in the induction tract as seats on a bus. When the bus pulls up to the stop, and people get on, in this case fuel atoms, only so many can climb aboard. The bus is going to a job site, where the strength of the people filling the seats is critical to the amount of work that can be done. People twice as powerful do twice the work, when you can only haul so many at a time to the job site.