It had something to do with humans that had evolved into bird/lizard-like people that were taller and lived in the trees. Also standard humans that were considered short and strong by the bird people, I think they called the standard humans dwarves.
I read it during the 90s when I was in middle school so I don't know if it's a kids book or an adult book. The book was in English, if that helps.
Humans have colonized a free-fall ("zero gravity") biosphere orbiting a neutron star. Some of the humans live in pseudogravity at the ends of enormous trees larger than skyscrapers, while others inhabit free-floating structures. In the absence of strong planetbound gravity, these colonists grow taller than their terrestrial ancestors. While their hands are still fully-functional, they can also use their elongated toes for holding onto tools and anchor points; in a way, they can "fly" and "perch" like birds. Only dwarves (the very shortest of the colonists) can use the advanced equipment built into the old spacesuits .
You can borrow The Integral Trees and The Smoke Ring as e-books from The Internet Archive.
