My problem is that A4 and other standard page sizes should follow the sizes of the paper - but they don't.

A4 paper (210x297) should be Width 29,7 and Height 21. But instead it is 27,517cm in width and 19,05 cm in height.

Why is this so?

Hi,

Do you mind sharing us a screenshot about the "page size"? As far as I know, we can set the slide size in Design tab, have you set the "Maximize option"? Please see this article:

https://support.office.com/en-us/article/Change-the-size-of-your-slides-040a811c-be43-40b9-8d04-0de5ed79987e

If I misunderstanding something, please feel free let me know.