PostgreSQL memory usage

Hello.

After looking at my DBMS server for some time I've understood that I don't understand what was going on...

A server has 48 GB RAM. shared_buffers is set to 12GB, work_mem - to 32MB, pgbouncer in transaction mode is used to
connectpool (pool size 80) to PostgreSQL 10.5 server.  

I see that at some point several postgresql backends start consuming about 16  GB RAM. If we account for
shared_buffers,it meens 4 GB RAM for private backend memory. How can we achieve such numbers? I don't see any
long-running(or complex) queries (however, there could be long-running transactions and queries to large partitioned
tables).But how could they consume 512* work_mem memory?    

С уважением,
Александр Пыхалов,
программист отдела телекоммуникационной инфраструктуры
управления информационно-коммуникационной инфраструктуры ЮФУ