What increases from mass or volume, when density increases?

Density of a substance is the ratio of its mass and volume. In other words,

density = mass / volume

Since mass is commonly measured in the units of g and kg, volume is generally measured in the units of ml, l, m^3, cm^3, etc.; the commonly used units of density are g/ml, g/cm^3, kg/l, kg/m^3, etc.

From the relationship between density, mass and volume, it is clear that density can decrease in only two cases:

a) mass decrease or b) volume increases. 

Since mass is the measure of matter contained in a body, it is a constant (unless some addition or subtraction is done from it). Volume, on the other hand, can vary.

Thus, density decrease can only take place when the volume of a substance increases. 

When the volume increases, the molecules move farther apart from each other and the amount of matter in a unit volume decreases, which means the density decreases. When we heat a substance, its volume increases and correspondingly, its density decreases.

Hope this helps.