hi umer syed..
It is already discussed by Alistu in one of the previous post that Eye opening is a characteristic that is used to evaluate system performance. Wider eye opening makes the system performance better by making decision making easier. And increase in power is a way to make system performance better and its effect can be seen in the eye diagram.
The increase in the power may cause system performance to degrade due to fiber non-linearities which come into play at higher powers.so the increase in power has a negative effect on the eye opening resulting in degradation of the eye opening which is known as power penalty as increase in power resulted in penalty or degradation of the eye opening.
So i think you got negative eye penalty because of this thing.
Hope it helps
with regards