the difference has nothing to do with size as is commonly thought!
this is misleading though. I don't think anyone could really know at which point the word was changed to just being used to describe a "smaller vessel" and saying now that size doesn't matter isn't accurate at all because it is.....very important, now. And has been for a very long time.
yeah royal navy is probably different and I wouldn't be surprised if they had a different word for a smaller sized vessel but they should still know though because international seafaring law is pretty important, but very confusing sometimes.