PyQt5 QPrinting setFullpage doesn't works

[Saelyth]

Hi, newbie on the forums and also newbie at Qt programming.

I am here because I have a problem, (which I researched for a fix and I am aware it's the same issue as This one on C++) and I can't find a way to fix on Python 3.5.

The issue: When I try to print my widget to a PDF the result is this one:
It should fit the entire A4 page but it resizes it to that small section (yeah, that red rectangle).
MzSHRX75RRSmfkvcgucxLQ.jpg

The code:

from PyQt5.QtGui import QPainter
from PyQt5.QtPrintSupport import QPrinter
 
# Print options
printer = QPrinter(QPrinter.HighResolution)
printer.setOrientation(QPrinter.Portrait)
printer.setPaperSize(QPrinter.A4)
printer.setPageSize(QPrinter.A4)
printer.setPageMargins(15, 15, 15, 15, QPrinter.Millimeter)
printer.setFullPage(True)
printer.setOutputFormat(QPrinter.PdfFormat)
printer.setOutputFileName("testfile.pdf")
 
# Render/Paint it
painter = QPainter()
painter.begin(printer)
MyWidget.render(painter)
painter.end()

from PyQt5.QtGui import QPainter
from PyQt5.QtPrintSupport import QPrinter

# Print options
printer = QPrinter(QPrinter.HighResolution)
printer.setOrientation(QPrinter.Portrait)
printer.setPaperSize(QPrinter.A4)
printer.setPageSize(QPrinter.A4)
printer.setPageMargins(15, 15, 15, 15, QPrinter.Millimeter)
printer.setFullPage(True)
printer.setOutputFormat(QPrinter.PdfFormat)
printer.setOutputFileName("testfile.pdf")

# Render/Paint it
painter = QPainter()
painter.begin(printer)
MyWidget.render(painter)
painter.end()

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Assuming This documentation is accurate, the setFullpage on true should fix it. so what am I missing?