When I open the same project in Scrivener 1.9 and Scrivener 2.9, the font size in Scrivener 2.9 seems to be much bigger.
But font size and zoom level are exactly the same in both versions. (see screenshot)
The character count of the files is different, Scrivener 2.9 counts more characters than Scrivener 1.9 (see screenshot). Perhaps this comes from German Umlauts? BTW, Scrivener 2.9 is right, I just checked it with my Text Editor!
In Scrivener v1 text scaled to 135% is equivalent to 100% scaling in Scrivener v3. I would not go into technical details, but this should explain what you see. This has been changed and improved to handle properly displays with different DPIs, including High DPI displays.
Which has the advantage of not being ant-sized on Linux. 
Is there a possibility to change the zoom level of all documents in Binder at once?
The zoom is per editor, rather than per document, so changing it in the editor footer will affect all text documents loaded there.
