It's weighted by size, so the same event will destroy Catholicism and do nothing to fervor of heresies. Which makes little sense.
If there's one naughty bishop, it should have a bigger effect in a small religion with only a couple bishops, rather than one huge one where the naughty bishop is just one out of thousands.
I think the default setting should be historical (hence we don't really see successful heresies), but then there could be a settings toggle to increase heresy/fervor loss fo Catholicism etc...