It is interesting to note that at the beginning of the piece of R’ Dustai ben Yehuda (on 83b), when we talk about different sizes (a small eye and a big eye), the Gemara says that if the one who knocked out the victim’s eye had a smaller eye than his victim, we would lack a fulfillment of “an eye for an eye” if we would knock out the assailant’s eye. Only when we speak of the possibility of one person paying (ie where the assailant’s and the victim’s eyes are different sizes) and another person having his eye knocked out (if their eyes are the same size) do we bring the passuk that has to do with “one law shall be for all.”
When the Gemara later brings the contrasting cases of a midget killing a giant and a giant killing a midget, one would have expected the Gemara to bring the passuk of “a soul for a soul” which indicates that the person being killed must be the same size as the one he himself killed. Instead we bring the verse of “one law shall be for all.”
There are two possible explanations that come to mind.
One is that the Gemara is assuming that you know the back and forth that was just stated, and it is almost like a חסורי מחסרא that you can add in, “if they are different size people than perhaps we would pay value in such a case. But then we would have the problem of ‘one law shall be for all…'”
Another possibility is that the Gemara is trying to show that we are not focusing in on the thing that was superficially lost (the body or the eye), rather the function of that thing (life/sight). We could not bring the verse נפש תחת נפש – “a soul for a soul” because at this point we are coming to prove our point by exclusion, and that verse actually implies our point directly, that we focus not on the body, but rather the soul. This second explanation, however, could be easily refuted because the word נפש, while traditionally translated as ‘soul’ is really referring to the animal soul, and more directly indicates the body itself, not only its life force per se.
ונראה יותר כפשט הראשון
I think it also interesting to note that in the הוה אמינא of the Gemara we would have thought to say that we don’t focus on the loss of vision (in the case of the eye) or the loss of life (in the case of murder), but rather on the eye itself or the body itself. Why would we think such a thing? I think that it is a very fair assumption because the Torah is מחדש that all you are paying is the actual value as a slave, not the intrinsic value of the eye or the life (which would seem to anyway be priceless). Thus perhaps we would think that we should be very ‘superficial’ as it were, and make a distinction between different size bodies and eyes. The חידוש is that we do not look at it so superficially, rather we look at the eye for its function, which is sight, and the body for its function, which is life.