The adjective proportional describes something that has a relative size or amount to something else. When you're a mom, you know it's best to make sure chores are given out proportional amounts according to age and size of your kids.
The United States has proportional representation in Congress, meaning that Congressional districts are divided according to the population, set after each census. The adjective proportional can also be used to describe something that has a constant ratio. This usage is especially common in math to describe things like proportional triangles or proportional variables. Proportionate is often interchangeable with proportional.