Size was a great heuristic for health for the entire history of the world up until maybe 50 years ago. If everyone's growth is stunted some to malnutrition then the biggest people are the least malnourished.
It doesn't hold as much these days but it still "feels" right and useful.
Like many statistical tools, the height charts doctors use are gaussian distributions and the inferences drawn from where an individual falls on that distribution do not boil down to "bigger === better."
It doesn't hold as much these days but it still "feels" right and useful.