Brush size changes without input

[Ofnuts]

In the case at hand this is the usual answer to the question about the brush having a random or unexpected behavior, which happens to be a very frequent question... You will note that we are taught later that the number are not so random, and so can then provide a correct answer. A good answer somehow depends on an accurate question(*).

Nice to see that you took the trouble to register here to share your opinion.

(*) Which is also why it is important to register your OS and Gimp version accurately...