In my main world I have a cave system right next to spawn, and until today I thought it to be large, but not massive. Thats changed, I wished to create a mob spawner, the classic dark room one we all know and love, and for maximum efficiency you have to light all surrounding caves up. I light alot of this system in past trips, but today I needed to light up all of it. When trying I noticed the vastness of the network. Twists and turns, areas repeating back on each other, and the general ease to get lost. The worst area was this area that came off the 2nd ravine in the system and it just seemed to go on forever, i don't think I lit it all up, thats how big it was. In this video: , you can see the shallow parts of the system. I can't find a program to check the caves of the world, so maybe this isn't as giant as I think, but to me its very very similar to a cave in my 1.0 survival world.
Large 1.15.2 Cave