I think regular human size can vary a lot by level of resource consumption - the principle being the larger the mother, the larger fetus and so the larger the resulting adult.
Obviously, if a height increase/decrease trend continued for a while, one assumes genetic variations which aided this would be selected for but it seems that the change itself doesn't necessarily need genetic as such.
From my observation, I notice that a lot of younger generation Chinese adults are likely to be taller than their older fellow country people as well. I have many Chinese coworkers and even the female coworkers are at least 5 feet 5 inches or taller (for young male coworkers, they are usually at least 5 feet 8-10 inches). Nutrition certainly plays a big role in determining one's height.
Obviously, if a height increase/decrease trend continued for a while, one assumes genetic variations which aided this would be selected for but it seems that the change itself doesn't necessarily need genetic as such.
See height increases in Japan: http://www.nytimes.com/2001/02/01/world/tokyo-journal-the-ja...