As the mass of a vibrating body increases, its frequency decreases, but as the tension increases the frequency also increases.
Mass and Tension: Any object that can vibrate is capable of creating a tone. Two factors determine the frequency of the vibration: the Mass of the object and its Tension.
As the Mass is increased, the frequency and hence the pitch decrease. This is quite familiar to most of us. For example, the thick strings on a piano or a guitar produce the lower tones.
Men in general have more mass in their vocal folds than women and hence, have lower voices.
On the other hand as the Tension of a vibrating body is increased, the frequency increases. This also is quite familiar to most of us who have plucked a guitar string while turning the screw to increase the tension of the string. The pitch rises as we turn the screw.