How to show large icons in Qt QTreeWidget?

I want to show large icons on items of type QTreeWidgetItem added to the QTreeWidget object. When I try creating a sample in Qt Designer, it easily shows the large size of the .ico file set to the items (based on the size I set for the IconSize). But, after checking the code behind using Form|View Code command and immitating the same behaviour, I fail each time to show the bigger icons.
Does anybody know how I can make it? My '.ico' file includes a wide range of sizes from
32x32, 64x64 to 256x256.
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