If the window is sufficiently small, the volume slider is compressed in such a way that it becomes unusable. If the user try to use it in this state, it will effectively slide the slider to the minimum, reducing the volume to 0. The "compressed" slider cannot be used in this state otherwise.

Sliding the "compressed" slider to the right or to the left will have the same effect, and slide it only to the left.*

Normal size

Smaller but still usable

Here the slider is unusable, but still active.