We don't know all the exact values yet, but there is (supposedly) a third value as well: minimum mass.
The efficiency changes linearly between these values, so let's say for the module you've chosen the max modifier is 1.5, the min is 0.5 (which means 1 at optimal mass)
Meaning that at minimum mass it will be 1.5 times more efficient, and at maximum mass it will only perform half as well as it does at optimal mass.
Problem is, we don't know the modifier minimum / maximum values and minimum mass values for everything. For shields though, edshipyard does a pretty decent calculation for you.
Hopefully this guesswork will change at some point in the future as they are planning to add more information on the outfitting screen.
Until then, make sure you are below the optimal as much as you can (not always possible) and you'll be fine. And remember for shields it is the (fixed) hull mass of your ship that counts, for thrusters however the cargo matters a lot (you can actually feel that in bigger ships when turning - trying to turn

).