Does this happen when you spam the primary fire or always?
You can see this in the video 3:21 I don't spam any button and yet the ball bounces off and later you can see that I make it shrink all the way down because earlier when this happened I found out that this helps. But I've been spaming sometimes and this would also happen. For me it looks like it happens randomly.
I can only blindly guess. This problem could happen if you have code like:
if (size+change_in_size < max_size) expand;
but the initial value is bigger than max_size - 2 * change_in_size. (because it happened with clean loaded level).