The fact they define the fluence as Joules per square centimeter implies the spot size doesn't matter. Lowering from 18J/cm^2 to 11J/cm^2 reduces energy delivered per square cm regardless of spot size chosen. 11J/cm^2 will obviously be less effective at vaporizing hair than 18J/cm^2.
In other words, if the spot size covers 1 square cm at a setting of 11J/cm^2, the machine must emit 11J of energy. If the spot size covers 2 square cm, the machine must emit 22J of energy at a setting of 11J/cm^2.
Also, larger spot sizes have deeper penetration because of how light refracts under the skin. So even if they switched to a smaller spot size and kept J/cm^2 the same, smaller spot will be less effective on deeper hair. Here's a research study
showing larger spot size was 10.3% more effective. So I would ask to switch back to larger spot. If they won't, at least switch back to 18J. Either way, your tech is either trying to cheat you by claiming 11J smaller spot is the same as 18J larger spot, or they need to be educated how their machine works.