Maybe the value of 'fringe-indicator-alist' should be frame-local?
Frames can be moved from one display to another with a different dpi.
>> Funnily enough, the current fringe bitmaps are too big for me,
>
> Too big as in "twice as big as expected" or in "too big for my taste"?
I use a small 10px font, so the customized bitmaps fit the font size.
Ah ok, great, so that's not a scale factor issue.
So maybe fridge bitmap size should depend on the font size, not scale?
10px fonts are not 10px at other scale factor than 1x. In general you don't have to change the size of your fonts when duplicating your screen resolution, even if your fonts were given in px size, because the toolkit scales them for you under the assumption of some standard ~96dpi base which allows pixels to be treated as something more than a "number of tiny dots, whatever their size is" measure.
What can be done is to adjust everything else in emacs to the effective (not nominal) pixel size of the default face (then scale factor would be the effective to nominal pixel size ratio of this font). I believe something like that is done in some places, it's sensible given that emacs is mostly a grid of characters. But anyway this is clearly not the approach taken for the fringe and some parts of emacs have geometry not so tightly coupled to font size, although I would indeed expect high correlation. And some toolkits (for example, gtk) offer a separate scale factor for font size, which is then applied on top of the general scale factor and seen mostly as an accessibility feature.