I have to post this as an answer rather than a comment (as I don't have enough rep), but you can use
awk to find the particular line of text you're looking for and replace the default value with whatever value is needed to zoom to your liking. I looked at my
Preferences JSON file, but I didn't seem to have the
default_zoom_level key, so I can't tell you exactly how to do it, but I did some tests on a sample file using a variation of this script:
cat PATHTOCHROMEPREFERENCES | sed s/ORIGINALVALUE/NEWVALUE/ | pbcopy | pbpaste > PATHTOCHROMEPREFERENCES
ORIGINALVALUE is the value is the default 100% zoom and
NEWVALUE is whatever you need to enter to zoom to 125%. As I mentioned above, I didn't see the
default_zoom_level key in the file I have, so I cannot verify that this will work in this specific file.
sed command used here should be able to and replace whatever text you want, but I suspect you will need to use
awk as well to find the specific line you're looking for, since there seems to be a lot of lines containing
default_ in the file.
There is probably a much more elegant way to do this, but I hope my answer helps!