Suppose there twice as many molecules in the same volume- the density is then doubled. IF the molecules move at the same speed, or, equivilently, if they have the same temperature, then, to a close approximation, the number of collisions will double. This measn teh pressure will double, too. So pressure is proportional to density. Triple the density, triple the pressure. The density of air can also be doubled by simply compressing the air to half its volume. We increase the density of a balloon when we squeeze it, and, likewise, increase air dentiy in the cylinder of a tire pump when we push the piston downward.