It seems there are two independent, equally influential values those control mathematica notebook magnification.
Just like volume bars in a certain media player(program) and volume bar in tray bar(right side of windows 10 taskbar).
One is the following, the familiar one. :
We can get the value by
CurrentValue[Magnification] 1.25 (1.25 means 125%)
(Note that if you see 125% in the lower-right corner of mathematica but the value isn't 1.25, you might have already touched on the more unfamiliar things that will be explained next.)
But I do not know how to change the value by executing a code. <- This is the first question.
The other is less familar. By default, the value is 1. You can change the value by executing the code
SetOptions[$FrontEndSession, Magnification -> 1.3] (* 130% *)
But I do not know how to identify(=figure out) the value. <- This is the second question.
If we set these two values to 125% and 1.3, then the final magnification becomes 1.25*1.3 = 1.625.
Note that it seems neither the two values can change the final size(we see by our eye) in image in output cell.
See : Creating a size changing image in output cell