1. To visualize the problem without a debugger or memory limits, add the attached patch.
   It just adds a qDebug() to show which value is being passed into QByteArray::resize().
2. Build Qt with that patch.
3. Build the attached project on this build of Qt.
4. Run the resulting program with the attached input.
   You'll see:

   Allocating 1
   Allocating 2147483641
   

   Allocating 2 GiB of memory doesn't seem appropriate for an 8 byte file.

This is a regression from Qt 5.15. There, you'll only see:

Allocating 1