Well, since Chewbacca has two arms, that appear to have the same range of motion as a human, he would have some form of pectoral muscle. This is just a mechanical requirement for the range of motion shown in the movies. And yes, I’m ignoring the human in the costume, it’s called suspension of disbelief. As the Holiday special showed, Wookiees are unlikely to have litter births. Except that’s not canon, and the twelfth issue of The Official Star Wars Fact File, released on March 20, 2002, says that female Wookiees have 6 breasts. Everything I can find says that document is still canon. This means that litter births are likely a thing. Scenes in the various shows and movie have briefly show Kashyyyk, there are never enough children shown to justify litter births. You’d expect a lot of identically aged siblings, and what is shown are not many children at all, and almost all of them alone in their appearant age group. This leads me to one of two conclusions. Either the extra breasts are vestigial, or even with advanced technology, only a few Wookiee children per litter survive infancy. The question is, why would Wookiee infants not survive into early childhood?