Masseter Botox: can it be limited to specific portions of the muscle such that slimming/proportions are controlled? (Photo)

The masseter muscle is a fairly large, bulky muscle, and a sufficient amount of Botox or Dysport must be injected to relax it uniformly. Botox and Dysport also have a certain zone of spread, depending on the dilution used, which means there is a gradient in terms of the most effect being seen at the site of injection, with a ring of decreasing effect around it. To answer your question, portions of the muscle may be preferentially slimmed - for example the upper half vs lower half, anterior half vs posterior half - but greater precision than that is not possible with Botox or Dysport for masseter reduction.

Unfortunately the GIF is not working so we only see the morphed image.
Yes, toxin can be localized to achieve a region specific effect.
However...the region is fairly unpredictable due to potential diffusion or spread of the injected toxin and therefore the result is not as precise as compared to what we can sometimes achieve with fillers.