Imagine 70% of all applicants being Indian nationals. Randomly sample all the applicants and with a big enough sample size, the distribution will be the same, i.e. ~70% of your samples will be Indian nationals.
Countries that have the most people in the world have the most people in the H-1B application process as well. China and India are my guesses, without looking at the publicly available data on H-1B applications. You don't see as many Chinese people speaking about it online because they, on average, don't speak English as well as people from India.
https://www.uscis.gov/sites/default/files/USCIS/Resources/Re...
This is FY 2011-2012, not sure if there are more recent PDFs.
https://www.foreignlaborcert.doleta.gov/performancedata.cfm#...
170+ MB .xlsx. Tried doing some stuff with it on a 12 core 64 GB RAM workstation. Took way too long and LibreOffice crashed a few times.
Google Sheets doesn't want to touch it even.