Here's why:

* Density: Density is a fundamental property of a substance that relates its mass to its volume. It is defined as mass per unit volume:

* Density = Mass / Volume

* Constant Density: For a given substance, the density remains constant. This means that if you increase the mass of the substance, you must also increase its volume proportionally to maintain the same density.

* Direct Proportionality: Since density is constant, mass and volume are directly proportional. This means that if you double the mass, you double the volume; if you triple the mass, you triple the volume, and so on.

Example: Imagine you have a block of iron. If you have a 100 gram block, it will have a specific volume. If you double the mass to 200 grams, the volume will also double.