That is interesting. I was under the impression (ok, I assumed and never gave it much thought) that they were all equal parts. Since a murder had to take place, I just assumed that the act of such a dark deed was where the power to make the horcrux came from. Yes it comes from the soul of the murderer, but also from the dark art act of murder. Maybe the murder is the energy (evil energy. Maybe part the horcrux also comes from part of the soul of the person that was murdered....Of course, I probably don't have the faintest idea :)
Actually, although the idea that each successive Horcrux would contain a smaller and smaller portion of Voldemort's soul is gleefully logical, the book actually leads us to believe that each Horcrux contains an equal portion of the soul.
Dumbledore says, "However, a withered hand does not seem an unreasonable exchange for a seventh of Voldemort's soul. The ring is no longer a Horcrux." (see the bottom of page 503)