At a given temperature, denser materials generally contain more energy, so heavier pans will cook food faster. On the contrary, more energy is needed to raise denser materials to a given temperature. At 120 °F (48.9 °C), meat begins to slowly soften as the protein myosin begins to coagulate and the connective tissue in the meat begins to break down. This also causes the meat to harden as the protein contracts.
As the temperature increases, so does the rate of softening. We can see the effect by looking at the temperature at which the water boils. The water will boil at 212 degrees Fahrenheit (100 degrees Celsius) at sea level, but will boil at much lower temperatures as the elevation increases due to lower air pressure.