Radiofrequency is not the right treatment for master reduction.
The masseters are a pair of muscles. If you're talking about muscle bulk reduction, injection of botulinum toxin will be the correct treatment. However, I will recommend adding energy device treatment like Ulthera to the jowls after botulinum toxin to the masseters to prevent excessive sagginess at the jowls/cheeks for individuals over 40 years of age.
For radiofrequency treatment, it can be used to reduce cheek fats with treatments like AGNES as the radiofrequency needling treatment helps to burn some of the cheek fats away. However, this usually requires 3 sessions to give good results.