For Grass Knot and Low Kick, which are also based on weight, pokemon over 440 lbs take 120 base damage and pokemon over 220 take 100 base damage.
Heat Stamp may be along the same lines. Emboar's weight is 330.7 lbs (150.0 kg), which puts him in the second class down. So it could be base 100 damage, but we don't really know just yet.
