James said in another thread:
Larger probes will make the treatment both more comfortable, and more effective.
Boy can I attest to this. 
I had another treatment today and it was killer…whereas last week I hardly felt anything at all. She didn’t think she was doing anything differently so we figured my hormones must be causing trouble, or maybe it was related to what I had eaten or something. Then when she was done and writing down the treatment info she noticed she had used a larger probe last week. I assumed she was using a smaller one because she thought it would be more effective…I suppose it was because it’s easier to insert? We plan to go back to the other one next time!