QImageWriter Makes Large PNG

The attached example program writes a PNG file that should be 100KB, but QImageWriter writes a large 118MB file that doesn't seem compressed.  QImage::save does the right thing. Qt 5.9.6 works fine, but 5.15.3 is broken.  QImageWriter::setQuality does nothing.  To be consistent with QImage::save, I would expect:

"The quality factor must be in the range 0 to 100 or -1. Specify 0 to obtain small compressed files, 100 for large uncompressed files, and -1 (the default) to use the default settings."