Guineafowl never give butchering returns, they're just too small.
From my experiments in egg hatching and butchering, newborn peafowl and geese are newborn size for about 3 seconds, then they jump to a random size with a maximum slightly less than adult. Bottom end runs about 20% of max.
This means that adult breeders raised in the fort could squeeze out a baby that is 5 times larger than their mom.
Man, I wish Goombageek was here to draw this.
Turkeys and chickens jump to a much smaller size, less than half of adult size. Too small, in fact, to butcher for anything. So there is some correlation between growth rate and final size. Perhaps turkeys, chickens, geese, peafowl, dogs, and cats all jump to a size based on their [BODY_SIZE:1:0:XXXXX] tags, the size they should be after 1 year.
I am terribly curious to find out how big baby cave dragons are. And I am running dfhack, so I'm curious if this can be reproduced in vanilla. But it's not going to be me doing the testing. Been wrestling with this bug all week. I am scienced out.