Your cave will be consisted of multiple points. Each of these points has a size associated with it. Now you will have line segments connecting them. Imagine these points and lines as metaballs and metacylinders. This approach will give you a basic shape for your cave.Now you might want to start adding variation. You may want to try randomly offsetting vertices.

This is my approach to this problem and there may be others which are better.

You could model your cave as a sequence of points, each with an associated size, with lines connecting them. Then treat each point and line as metaballs and metacylinders. This gives you a basic shape for your cave, to which you might want to start adding variation, such as by randomly offsetting vertices.