According to Dr$.$ Taylor, as trucks are filled up$,$ and they drive say $1000$ miles to deliver the load, costs of the trip:

Go up in proportion to the increase in weight on the truck

Costs go down in proportion to the decrease in weight on the truck

Costs are for the most part fixed and do not go up much as the weight on the truck is increased

Go up in a variable and very irregular way

Costs go up in direct proportion as the cube of the truck that is filled go up