Actually you understand it wrong.
Taken from here
Low Kick inflicts greater damage on heavier opponents, as per this table:
Weight (lb.) Power
0-22 : 20
22-55 : 40
55-110 : 60
110-220 : 80
220-440 : 100
440+ : 120
So yes, it inflicts a set damage.
It is not set damage it effects the base power just like Grasss knot. It is not set damage even if it is 120 it could still do almost no damage to a high level Pokemon.