Is it true that your head grows proportionally equal to your body as you get older?

Not exactly. Weight varies by height, tall people usually weigh more than short people, and a person's weight does grow proportional to height as you grow. The head size is not related to height or weight. You can have a big head if you are tall or short. But, if you have a small or big head, your relative head size often remains the same as you grow. That is, large heads remain large, for example.