It is called operating leverage.
Revenue grows at x%, and profit grows at (x+ ol)%
If costs are fixed & there is extra capacity extra sales go mostly on the profit side and change the % profit drastically. Think of a school 10 students per class, 15 students per class minimal increased cost, still 1 teacher, same # hrs same class room, more desks. last 5 students almost pure profit.
Machines not idle, cost of supplies, the rest profit.
The author is really confused on basic economics.