No, 8 mo. definitely isn't adulthood.
I was always told from day 1 of bringing Aaliyah home (from my breeder and from the breeders on here when I posted the question) that I shouldn't let my pup be rolly, polly - they need a little extra weight when they are very young (in case they get sick), but generally you measure their weight the same way - by being able to find the ribs when you feel for them.
But back to the 8 mo. age, Liyah's body is starting to look like an adult now, and her breeder thinks she is the perfect weight for her size, and although she isn't as lean as my adults, she definitely has clear waist definition, clear tuck-up and you can find her ribs easily - but again, Liyah is being shown.
Hopefully Pat, Lisa or Andrea will see this thread and chime in…they know a lot more about it than I do - especially from a breeder standpoint.