@kobat It is currently not possible to change it by default.
We have the configuration to set the default value in JS pzpega_ui_template_grid_helpers
gridMetaData.rowHeightClass = gridState.rowHeightClass ? gridState.rowHeightClass : ( rowHeightClass ? rowHeightClass : pega.ui.tGridConstants.GRID_ROWHEIGHT_COMPACT);
This JS rule type is final, so we can't save in client application rule set.
This issue has been logged as a Product Enhancement (FDBK-83645).