Well it's open-source, so you can have a look:

https://github.com/kiprotect/klaro

It uses Preact and a few JS helper libraries and includes translations for more than 10 languages currently. It also includes styles by default (though you can get a version without styles). The compressed size is 37 kB which I think is reasonable, smaller is of course better. We haven't prioritized size for now as it's comparable to a PNG/JPG and usually negligible on most sites, we might look into this in the future though.