How can I print this PDF and fill an A4 page?
It is not really a glitch.

The accuracy of print preview depends on 2 factors.

One is the Printer it self. 

Try changing printer type or select high print resolution (1200 DPi or more).

The second factor is the application showing the print preview.

If you for example use the Adobe Reader it shows much better print preview then Preview.app it self.

Example: