Why is my postgres `base` directory filling up rapidly?
To generate 10k WAL files overnight, your database is obviously very busy.  It is not surprising that the non-WAL of such a busy database might also grow rapidly. Indeed, why wouldn't it?  If your database were that busy but not growing, that is the weird thing which would be in need of explanation, based on what you know about how your database is used.

Do you have hot_standby_feedback turned on?

If your database is still up, you can use these queries to figure out where all the space is going, although I usually just start with '\l+' and '\dit+' in psql (those ones don't sort by size, so if you have a lot of objects to scan through, the other ones may be better).  If your database is not up, you can use OS tools like 'du' and 'ls' to to figure out where the space is going.  But translating file names back to table names without a running database will not be easy.