Are these Coronula diadema and Conchoderma auritum? I hardly know anything about barnacles, but the ones shown here certainly resemble those two species. I just don't know if there are other possibilities given the context. The Coronula (the ones that look like "normal" barnacles) were about 10 cm in diameter, I think, though it was kind of hard to tell, since they were on the chin of a living humpback whale. Observed near the Farallon Islands, ~60 km west of San Francisco, CA, USA.
Also, I've read that Conchoderma is merely a commensal epibiont on Coronula, but it really looks like it's bursting out of the inside of some of these barnacles, which seems somewhat less than harmless. Is this just a weird angle, or is Conchoderma also a parasite?
FYI, the above was mostly written back in 2009 when I took these photos. There's lots more info and research in the comments at https://www.flickr.com/photos/ken-ichi/4007914560/