"Direct proportion" means that as one quantity increases, the other quantity also increases in a predictable way. If you have more of one thing, you will have more of the other thing as well. Full definition
An animal's daily energy use usually increases in direct proportion to its mass. (newscientist.com)
Use in direct proportions wherever a recipe calls for milk. (mindbodygreen.com)
He also noted that the intake of extra water increased the elimination of silver in the urine in almost direct proportion to the increase in water intake. (clspress.com)