On the "they were shorter back then" theme. A possible explination I have heard is that there was a great deal of discrepency in the diets of people from diferent classes. Quick note, I am not an archeo-nutritionist, so take my paraphrasing awith a pinch of salt. Your average mediaval peasant was probably eating a diet of turnip, onion and other starchy , only moderately nutritious vegitables, and probably in smallish portions. End result, short, malnurished peasants. The ruling class, on the other hand, seemed to have had a steady diet of Meat, meat, meat, cheese, meat cheese and meat, i.e. high protein and high calcium. End result, larger aristocracy, with a tendency to develop gout.
Taking this view, the size of the harnesses isn't a good indicator of "average" size of people, since those turnip eating peasants were not likely to have a full suit of high gothic plate made for them.
Perhaps a bit like trying to estimate the size of 18th century Japanese from the Obi of champion Sumo wrestlers.

Another reason is skewed statistics due to high infant mortality.

If you calculate average height on basis of excavated burials, you will get a lower average. For people surviving the first ten years, the average height is presumably still lower but not by so much. Diet being the primary reason for us being taller today, and also some periods before the middle ages.

I've always heard the about 2 inches is what the difference amounts to, at least between people of the same place in modern and pre-modern times (assuming a fair degree of genetic isolation). Wasn't Edward IV supposedly like 6" +?

In regards to the wasp-waists, I doubt that those have any explanation beyond fashion, given that they closely paralell the cut of men's clothing at the time they were prevalent. Being from such a functional age ourselves we sometimes like to look for a utilitarian purpose in something that the people of the past just thought looked cool--then again, I could easily see someone doing something similar for modern material culture if it's unearthed in 300 years.
-Wilhelm

Speaking of height I read on medievalists that height actually dipped during the industrial revolution