Hi,
Under MacOS, digiKam use bundle as standard sand-boxing. All digiKam binary and library are is installed in /opt/digikam/ (almost 99% of the bundle).
Some links are registered in /Applications to be visible from launpad panel. That all.
To uninstall, Open Finder and go to Applications folder. Search digiKam directory, and move it to the trash. Empty the trash to really remove files definitively. Et voilà.
It work as expected here. /opt/digikam size is really 478Mb here:
macbook-pro-de-gilles:digikam gilles$ pwd
/opt/digikam
macbook-pro-de-gilles:digikam gilles$ ls
Applications bin etc lib libexec
macbook-pro-de-gilles:digikam gilles$ du
...
449840 .
macbook-pro-de-gilles:digikam gilles$
Note : remember that database and collections are untouched from this uninstall process.
Best
Gilles Caulier