A normal man has two arms, two arms and two eyes. The average man has just a little less, since some men have fewer than two arms and nobody has more. That illustrates the difference between normal and average and maybe some of the dangers of talking about averages. The median American has all the parts and is much closer to normal.
The average man is changing all the time. The average American is getting shorter and fatter. Shorter is a statistically artifact. As the ethnic mix of the U.S. changes, on average we get shorter. Consider how it works with immigration. The average Hispanic-American is almost three inches shorter than the average non-Hispanic American. As that population grows in relation to the total population, the average changes.
Becoming fatter is a matter of changing lifestyles.
I read an interesting articles showing the average American. Take a look. It is interesting.