By "not drawn to scale," we usually mean that you shouldn't make conclusions based on the drawing, because it may be a little off. But bigger things, like one circle being inside another, a line only touching the circle once, etc., are safe to assume.
In this example, you can assume that the radius of size 17 is drawn as the bigger circle. Otherwise, the diagram would be wrong, not just "not to scale." It's great that you are thinking about the questions with this level of rigor, though!
Just to clarify your question, when you say "line down," I assume you are referring to the line from the center of each circle to where the line touches each circle, correct? If they, your question is how do we know we can draw such a line, that's a great question!
One of the nice properties of a circle is that for any tangent line (that is, a line which touches the circle at exactly one point), it makes a 90-degree angle with the radius. So we can say that for both of those circles, a line from the center to the point of contact will make a 90-degree angle.
I hope that's helpful. Let us know if you need more clarification!