We also have to be careful about what we define as a 'corporation'. You're clearly putting non-profits under that category and that may be fair, I don't know, but for non-profit corporations that have a large portion of their income in the form of donations or have been created with the specific charter of promoting, developing or maintaining FOSS, it seems pretty disingenuous to lump them into the same 'corporate' category. It gets further complicated if you consider colleges and universities as corporations.
Here are some links that may help move the discussion, though they don't answer the question head on (as far as I can tell):
* https://www.quora.com/Has-anyone-ever-tried-to-estimate-the-...
* https://www.linux.com/news/study-open-source-worth-387-billi...
* https://octoverse.github.com/2016/
* https://thenewstack.io/week-numbers-data-lots
* https://www.techrepublic.com/article/who-contributes-most-to...
* https://redmonk.com/dberkholz/2013/04/22/the-size-of-open-so...
* https://www.techrepublic.com/article/for-50-percent-of-devel...
Some estimates put the Linux kernel at around $300B+. The above links measure contributions from various companies, of which Microsoft (who now owns GitHub) and Mozilla make the top 10. There needs to be some work done to figure out the actual answer the question ("do corporations fund/create the majority of FOSS?"), including figuring out what aspects of the question are actually valid, like whether it's code used or code created, what counts as a 'corporation' etc.
Like I said, I don't actually know but this is something that I had a bias towards believing that FOSS excelled in spite of corporate backing instead of because of it.
I'll say it again: citation needed.