Thickness always stretches the display range from 0 to the maximum pixel value (local thickness) in the image. Because images have different maxima, the same display colour represents a different local thickness in each image. What you have to do is set the display range to the same values across all the images.
You can do it with the macro command
setMinAndMax(min, max), like this:
myMaxValue = 60; //<- set your desired max value to be displayed in the LUT here
You have to run this macro for every image.
If you include a scale bar in only one image it will be true for all of them.
You can also create a custom scale bar of any size like this, which can be handy if you are arranging elements in a more complicated figure.
//Maximum thickness to display. Make sure it's the same max as applied to the images!
myMaxValue = 60;
//Width of the scale bar
rampWidth = 128;
//Height of the scale bar
rampHeight = 16;
//The ramp image contains floating-point values from 0 to 1 running left to right
newImage("Untitled", "32-bit ramp", rampWidth, rampHeight, 1);
//Multiplying values stretches them linearly from 0 to your max value
//Set the display range to stretch the LUT from 0 to your max value
//Apply the Fire LUT