Allowing people to attack someone 1/10th their size just encourages bullying. It is particularly difficult for someone who is just getting established to have someone who is well-established to start farming them.
You would not allow this kind of activity in real life (a 10-yr old beating up a 1-yr old, 10 guys vs 1, etc). Please don't allow it in the game.
The absolute max should be 2x.