When you say a size 2 probe may not have as wide of a “killing spread” what does that mean? Do thicker probes kill hairs better? Also does the pain correalate so that the smaller the probe the more pain or “sting”?
You got it. The smaller the probe, the hotter the sting (it is called “point effect”) and the large the probe, the more comfortable the treatment energy. That is one of the pluses of using a stereo microscope. One can use size 5 and size 6 Ballet Golds on everybody in almost every area of the body. I can’t remember the last time I had to go down to a size 4. 
What is a stereo microscope?
A stereo microscope is one in which there are two eyepieces, so that each eye sees the image, vs a microscope with only one eyepiece.
Stereo microscopes maintain one’s depth perception.