You can add "hugeness" by using the person viewing the artwork as the "ruler"; in other words, you need to play with angles to make the person viewing your art feel small!
Instead of making the art viewed at the height of the eyes, you lower this as if the person was small and add an angle upward. The more depth you can add with perspective, the bigger it will look.
You can make the viewer feel "wrapped" by your artwork too by making the drawing go above their head. And you can also crop tall building and not show its end (roof, cave ceiling.)
For caves, you can also use shadow, smoke, fog in the end of your drawing to give an impression of depth. You can use other natural elements instead of objects like rocks; the way you can represent depth is by using layers of rocks (eg. see 5th picture.) And you can use ray of light in the top of the cave to add some height. The ray of light will serve as ruler.
Same goes with leaves, roots or anything human can identify and already have a clear conception of them in their mind; either you make these elements giants to make the viewer feel in a world of giants or you use these elements very small to make them look far and use this to add depth.