Disclaimer: I find the BMI calculator to be flawed for outliers in height, both short and tall. But, find it equally erroneous and foolish to adhoc redefine an empirical formula based off what some would consider intuitive reasoning.

There is a fundamental problem with assuming that just because we are 3D that our mass is a cubic function of height. This assumption puts our height, width, and thickness all into a 1:1 ratio for growth which simply isn't true. (In fact, if you look at the history of the BMI they had acknowledged that a higher power was more accurate for development of children, but the 2.0 was selected to more accurately reflect the general adult populace.) But, back to the point. Generally, the human body predominantly exhibits vertical growth, if we were to grow as wide as we did tall from ratios we had as newborns we would all be planets with some hefty bone structure.

Take into consideration the following calculation from when someone last posted an attempt at a new BMI formula with just a 2.5 power:

I get the approach of redefining a curve around an approximate average height, but I feel like it still doesn't adequately capture the extremes. (I mean yay, I dropped nearly a point!) But, for an ideal body weight (a quick search yielded this report, accuracy to be further validated) a 4ft tall man should be around 81lbs which is nBMI = 29.1, and with small frame adjustment to 73lbs is nBMI = 26.2. Both are still overweight, whereas the original BMI calculation says they are both upper healthy. To further expand, the ideal weight of a 4' individual (calculated from a 2nd-order polynomial trend of the ideal weight curves of men and women from 42" to 78") is 77lbs, and would ideally put them around the 21.75 BMI in my mind (middle of the healthy weight range).

Essentially, we should have one rule for shorter than average and one rule for taller than average unless we wish to use a more complicated formula for BMI.