Hi,
That’s strange. Which 3ds max version are you using? The Paint tool uses a "default" 3ds max's brush - just like the Paint Deformation tool (where you can find settings for this brush under the Brush Options button). You should see these options and by marking the Enable Pressure Sensibility check-box you can set the Pressure Affects option to Size.
Thanks in advance for any further comments on this issue you can share about it with us / our user community (in case some user(s) will run into similar troubles).
Best regards,