Comment 18 for bug 1310316

I also have this problem.

It seems to persist the multiplication on every logout so the increase/decrease continues to be reapplied making it exponentially bigger/smaller each time - it's not random.

For people suffering with this issue, set the slider to the value you like, then log out and in, until it is the desired size. Then set it back to 1 and it will stay at that size.

I'm on a Dell XPS 13.