It seems that the only solution for the moment is to modify the SVG themselves :(

I've filled a bug report about this and also created an initial patch
https://qt-project.atlassian.net/browse/QTBUG-147536

Hmm it wont let me attach the SVG's

But I'll paste the svg.
Closed Arrow

<path d="M8 5.00009L2 8.66009V1.34009L8 5.00009Z" fill="#CCCCCC"/>
</svg>

Open Arrow
<svg width="10" height="10" viewBox="0 0 10 10" fill="none" xmlns="http://www.w3.org/2000/svg">
<path d="M8.65997 2L4.99997 8L1.33997 2H8.65997Z" fill="#CCCCCC"/>
</svg>

Looking at the images side by side I can see they are slightly different,
But in a way that is the opposite of how they are displayed ion the sample app!!

The png versions have the triangle touching the sides of the image,
However in the SVG's the triangles has some empty space around the triangle.
But in the sample the png version the arrows looks smaller, and the svg looks bigger?

the code...

#include <QApplication>
#include <QScreen>
#include <QTreeView>
#include <QStandardItemModel>
#include <QHeaderView>


int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // 1. Create the Tree View and populate data
    QTreeView treeView;
    QStandardItemModel model(4, 2);
    model.setHeaderData(0, Qt::Horizontal, "Task Name");
    model.setHeaderData(1, Qt::Horizontal, "Status");

    // Fill dummy hierarchical data
    for (int i = 0; i < 3; ++i) {
        QStandardItem *parentItem = new QStandardItem(QString("Project Phase %1").arg(i + 1));
        QStandardItem *statusItem = new QStandardItem("In Progress");
        model.setItem(i, 0, parentItem);
        model.setItem(i, 1, statusItem);

        for (int j = 0; j < 2; ++j) {
            QStandardItem *childItem = new QStandardItem(QString("Sub-task %1.%2").arg(i + 1).arg(j + 1));
            QStandardItem *childStatus = new QStandardItem("Pending");
            parentItem->appendRow(QList<QStandardItem*>() << childItem << childStatus);
        }
    }

    treeView.setModel(&model);
    treeView.expandAll();

    // Enable alternating backgrounds required by the stylesheet
    treeView.setAlternatingRowColors(true);

    // 2. Define and Apply the QSS Stylesheet
    QString treeStylesheet = R"(
        /* Main view styling */
     QTreeView { background: #4d4d4d; selection-color: black; selection-background-color: #f9d20e; border-bottom-right-radius: 4px; border-bottom-left-radius: 4px; show-decoration-selected: 1; }
     QTreeView::item { height: 25px; padding-left: 7px; background-color: transparent; }
     QTreeView::branch:has-siblings:!adjoins-item { border: none; }
     QTreeView::branch:has-siblings:adjoins-item { border: none; }
     QTreeView::branch:!has-children:!has-siblings:adjoins-item { border: none; }
     QTreeView::branch:has-children:!has-siblings:closed,
     QTreeView::branch:closed:has-children:has-siblings { border-image: none; image: url(:/ClosedArrow.svg);      width: 5px; height: 5px; }
     QTreeView::branch:open:has-children:!has-siblings,
     QTreeView::branch:open:has-children:has-siblings { border-image: none; image: url(:/OpenArrow.svg);      width: 5px; height: 5px; }
     QTreeView QLineEdit { color: #000000; background: #ffffff }
     QTreeView QScrollBar:vertical { padding-left: 3px; background: black; width: 11px; margin: 0px; }


    )";

    treeView.setStyleSheet(treeStylesheet);
    treeView.resize(450, 300);
    treeView.show();

    return app.exec();
}

I've been swapping out the PNG's and SVG's - each result in a different size of the triangle that get rendered, but neither SVG or PNG are effected by the icon-size or, width and height properties?

How am I supposed to get matched sizes?
The SVG file is near exact copy of the PNG image, why am i getting such different results?
and what is the correct way to control the icon size for this specific icon / image?

Regards,

Out of curiosity, what happens if your set the icon-size in your stylesheet ?