Broadly speaking, make sure that every ZBrush project is sized to the proper size (Tool> Geometry> Size > XYZ scale of 2) at the Start. This kind of distortion can happen when working on meshes that are abnormally large in the ZBrush worldspace. It’s the result of an improper workflow.
By default, or if properly imported into a tool with an Export Scale factor of zero, an imported tool should be auto scaled to this size. If it wasn’t, then it was improperly imported–probably on top of another tool with a non-zero Export Scale factor.
If working totally in ZBrush, unifying a mesh should be the first thing done, and then periodically as needed.
I don’t know if there’s anything that can be done at this point, other than trying to smooth out the mesh with Tool > Deformation> Relax or Tool > Deformation >Smooth.
You could also try using the basic command in Zbrush (Tool > Deformation> Unify) instead of Scale Master to see if it’s specifically an issue with the plugin and not the tool. Or you could try manually scaling the mesh to the proper size with Gizmo in smaller increments to avoid the distortion.