I took a 95 pound doe to a processor when I was busy, and got only 23 pounds of meat back. She said that was normal for an average deer.

Next deer I got was a 110 pound buck; I got 54 pounds of meat off him, plus I threw away a lot of shot-up shoulder meat.

So I guess a large part of it is how tight to the bone you cut the meat, especially in the legs and ribcage.
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It's not rocket surgery, for crying outside!