thats just the tip of the iceberg

Thats just the tip of the iceberg is another way to say that that is just a very small part of something that is much larger or more important than what they are referring to. This is most commonly said in response to someone speaking of just one small detail but not seeing the big picture. In a dilemma or issue which is very large and important, the small details or the tip of the iceberg because the dilemma is much bigger than those details.