By itself, single-element metals are highly malleable because the molecules of the metal are uniform in size, and this gives adjacent molecules room to move as each molecule acts like a ball joint to its neighbors.
Alloys are created by infusing a large-molecule metal with another element that has a much smaller molecule. The smaller molecules fill in the empty spaces between the large molecules, thereby making the metal seem more solid as the large molecules are fastened into place by the tight packing of small molecules in between the large molecules. Within an alloy mix, the same amount of space can contain more matter because previously unused space is now efficiently filled by smaller molecules.
Density increases. The two elements will inevitably occupy a space that is less than the space occupied by each element separately.
The heavy metal can be quite formidable by itself, but as an alloy, it can become more effective at achieving goals while maintaining a desired equilibrium.