QPixmap Bigger Than Same QIcon?

In our project the tree model returns a mixture of QIcon and QPixmap, all of which are created from the same files at the same resolution (18 by 18 pixels). I noticed that when high dpi scaling is enabled, the QPixmaps seem to be bigger which looked really odd being mixed together. So I created a reproducible and sure enough was able to reproduce it in a more standard out of the box tree model.

It seems to be directly influenced by Qt::AA_EnableHighDpiScaling as when this attribute is set in main before the QApplication is constructed, the tree view in my reproducer that returns QPixmap from the same file, instead of QIcon, looks way bigger.

This is not simply a matter of I consider this to be a bug, as I would expect them to all be equal in size, I also find it confusing. If this is not a bug at the very least I think an explanation would be helpful.