I haven't owned one yet. But I've been looking because my daughter will be ready for a 65 next year after 2 years on her KTM JR50 Pro. They are quick bikes, but it would seem an 85 would be better for a small adult bike. The KX65 has regular right side up forks and is quite low to the ground. It also has a pretty small shock, I'm not sure you could get springs that would keep it from bottoming out. 65s seem to go for way cheap, so that might be a motivating factor.
65s and 85s are not like some of the other air cooled trail bikes. These bikes are not made for adults to ride as a play bikes, they are made to be competition bikes for kids. They will not stand up well to the constant beating of a full sized adult.