why postgres process takes so much memory during day time

Hi all, is there any way to figure out why particular postgres process takes more and more memory during the day? This process performes order data queries coming from order management system - exchange system. Normally it's about 0,5mln messages per day but after aprox 4h postgres process occupies :

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                                                                                                    
11920 pgsql     20   0   17,6g   9,7g 566572 S   2,6 10,4   2:40.47 postgres

Perhaps dev guys (our team) should change something inside our system or it's postgresql conf matter? Postgres engine is 9.6.7. Any hint on this please?

Thx

Marcin