A Combination of the 2 previous messages did the trick.
First, I added
[X11]
ServerArguments=-dpi 192
That was not enough, however.
I then deleted
CursorTheme=Colloid-Dracula-cursors-dark
And tried again and this time it worked !
When I tried only the latter it didn’t work.
I guess I needed both the X11 setting AND that my cursor theme does not support that size. I now have a bigger cursor. Not the one I want, but it’s good enough.
Thanks for all people who replied!