I have this same issue. I can see that this has been a problem for many, many years and has not been fixed. Why isn’t bbpress not picking up the size from the wptheme? It makes no sense that everything else on the website has normal font size and the forum is miniature.
This seems like an easy fix for a developer but is a real pain for people trying to figure out how all of this works, but no one seems to want to bother fixing it.
If we have to add CSS, then we have to add a child theme. I am using Astra and a templet. I have no idea where to get a child theme for that. And for something like this, should we really have to? And I really am not familiar with CSS.
I beg you, PLEASE PLEASE PLEASE fix this. This stuff is hard enough for us newbies.