RedHat wasn’t always as big as it is now. In fact, it was likely smaller than Debian, in terms of manpower, for some time. I suspect the total number of package maintainers is still higher on the Debian side.
Probably at least partly by necessity. Not everyone participating is doing it as their FT job. I'm sure that RedHat is paying a lot of devs to work on these things as a full time job. Even just integrating software from others as packages, etc.