No expert either, but afaik you can divide livers into thirds and they will regrow. So when you transplant a liver you take two thirds from the donor, I assume to give the recipient more liver to work with, but I don't know why.
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Though, i have always been curious as to why the body only has one liver while having pairs of many other organs...
Perhaps it would be far easier for the donor to regrow the majority of their own liver in their own body than it would be for the recipient, given the immune response and other factors that I would imagine complicate a transplant.