"An eye for an eye is pretty much definition of proportional response is it not? The response is equal to the offense. You take one eye, one eye is taken from you".

The term, (that goes far back in time) in its purist form may seem to mean what you suggest. But, one should not assume that your "proportional" interpretation is shared.

I expect there could be, and likely are, many different definitions, and interpretations....depending on a perspective.

For example, it is likely that a historic leader may interpret the value of "his eye" or the lives of "his siblings" as being greater than the eye or sibling of others in that society. Historically, eye for eye revenge has seen hugely disproportinarte retaliation.
   
Some have also put forward, that in some of our current Western societies, that the perceived value of a citizen of one country is seen as much greater then the value of a citizen of another. It does not take much tinking to come up with examples.