Here is "Anthropometric reference data for children and adults: United States, 2011–2014" from the CDC.
https://www.cdc.gov/nchs/data/series/sr_03/sr03_039.pdf
Page 19: Table 15. Body mass index values for males aged 20 and over and number of examined persons, mean, standard error of the mean, and selected percentiles, by race and Hispanic origin and age: United States, 2011–2014
Most posters on LetsRun are probably in the Non-Hispanic white male, 20–39 years category. Here are the percentiles for this group. Most fast or serious runners have a BMI of under 21. If you have a BMI of 21, that would mean that 9 out of 10 other young white guys would have high BMI values than you. Even if you're Asian, that would place you under the 20th percentile.
Non-Hispanic white, 20–39 years (male)
5th 20.1
10th 21.1
15th 21.9
25th 23.2
50th 26.4
75th 30.6
85th 33.4
90th 35.9
95th 38.5
Mean BMI: 27.7
Non-Hispanic black, 20–39 years (male)
5th 19.5
10th 20.5
15th 21.4
25th 23.3
50th 26.4
75th 32.5
85th 35.4
90th 37.2
95th 41.3
Mean BMI: 28.4
Non-Hispanic Asian, 20–39 years (male)
5th 18.5
10th 20.1
15th 20.7
25th 22
50th 24.2
75th 27.7
85th 30.2
90th 31.4
95th 33.5
Mean BMI: 25.2
Mexican American, 20–39 years (male)
5th 21.3
10th 22.5
15th 23.7
25th 25.3
50th 29.2
75th 32.6
85th 35.1
90th 38.2
95th 40.1
Mean BMI: 29.7
Check out the CDC PDF report for more data on how Americans are shaped these days.