Heat Crash's base power is relative to a range between your weight and your opponents weight. To put it simply, if your weight is at least twice that of your opponent's, the move's base power will be 40. If your weight is at least twice but no more than thrice that of your opponent, its power will be 60. On the chart below, "r" represents your weight divided by your opponent's weight.
5 ≤ r = 120
4 ≤ r < 5=100
3 ≤ r < 4=80
2 ≤ r < 3=60
r < 2 =40
To answer your question, Pignite weighs 122.4 lbs. and your opponent (whoever that might be) weighs 37.8 lbs. Since the move is STAB and has a base power of (122.4/37.8 = approx. 3.2; 3 ≤ r < 4=80 ), you'll be doing damage of 120 power.