Asked in Titanic
How do icebergs become higher than the sea and why is the iceberg so big?
For each part of the iceberg you see above the surface, there's a much bigger part below the surface, providing buoyancy for the part that is lifted into the air. Just as with ice cubes, most will float below the surface and a little will float above.
The size - well, that's determined first by how they break off from glaciers and shelf ice. Later by erosion and melting.