Think of density as the chance you could point your finger at any area in a container and there is an object where you pointed.
Imagine if you had a container that you could shrink and expand at will, and you throw in a certain number of bouncy balls. Without changing the amount of bouncy balls, if you decrease the volume (shrink the container), there will be less space for the bouncy balls to move in, and therefore a higher chance that there is an bouncy ball at a point you choose in a container.
The density will increase if volume decreases. This can also be seen in the formula D=m/v : if the denominator of a fraction decreases, the result will increase, as the numerator is being divided by a smaller number.
The mass will stay the same if the volume decreases, unless you are specifically told it changes.
