Ram Usage doesn't release all after close designer or preview

[proton07]

Hi ~~~ i have noticed a problem
my software use about 36 mb or ram after opening designer it increase ram usage to 70 mb .. after closing designer ram decrease to 61 to 59 mb i knew if i open designer once again it never take more than 76 mb with all my software but it feels like something it never release
why ram usage didn't back to 36 mb or at least 40 mb ?
The problem in preview too
here's my code

report=new LimeReport::ReportEngine();
  report->dataManager()->addModel("tableview",ui->tableView->model(),true);
  report->loadFromFile(QDir::currentPath().append("/release/report/test1.lrxml"));
  report->designReport();

  delete report; 

// i tried report->deleteLater(); too
i am using version 1.6.0 windows 11 qt 5.15.2
Thank you