"It's just the tip of the iceberg" means that the problem is much larger than it appears. (Apparently the part of an iceberg that is visible above water is only a small part of the whole thing.)
Because or the density of ice (like a single ice cube in a glass of water) icebergs float with only a tenth of their mass showing above the water line. (Fresh water icebergs floating in salt sea water would ride a little higher than fresh ice in fresh water.)
The metaphor is popular because of the element of hidden danger. A very small
Why is there a small hole of the barrel of a non retractable point ball point pen?
Just a guess: When the pen is exposed to heat, the air in the chamber will expand, possibly enough to force some of the ink/paste out through the tip. It might ruin the pocket of your white shirt.