An array of parameter of n parameter choices will return values between 0 and n-1.
LUA arrays are indexed from 1 to n.
So, when the user selects 0.25 as the scale factor, the parameter value is zero and scaleFactors[params.factor] returns a nil value.
You will want scaleFactors[params.factor + 1]

Also, you will want to note in the description that this mod should be loaded after all mods that add cargo types or that otherwise change the cargoes that vehicles carry. Otherwise, your mod will not modify the cargo capacities that are added to vehicles by those other mods.