getSize() gives wrong values

[antxDotDe]

I have a PDF with 1300mm at 1800mm at 300dpi.
$pdf->getSize() returns for width: 612 and for height: 792
I expected something like: 15354 at 21260 (that is the png I get when $pdf->save() )

another PDF has 211,7mm at 139,3mm at 300dpi
$pdf->getSize() returns for width: 600 and for height: 395
I expected something like 2500 at 1646 (that is the png I get when $pdf->save() )

Do I interprete those values wrong? I can not tell from those values whether it is to large or not.