Why were Javelin fin flashes so huge?

Here are some other pieces of correspondence on the matter:

Bear in mind that the Harmonious Dragmaster squadrons needed a lot of navigators, and they didn’t have their radar until they were on board; they had to find their Flatirons quickly, especially in the dark, and they obviously couldn’t rely on their drivers, mt or airframe.
Not that I’m prejudiced in any way.        Mike the aged

Their arrival was heralded by that curious throaty howl, so typical of the Javelin, which was a combination of jet noise and an organ like note produced, it was said, by the air flow over the four large 30mm Aden gun ports in the wings. It was this unique noise which earned the Javelin its RAF nickname of the “Harmonious Dragmaster.”     Extract from a Gloster article.

It was very noticeable on start-up, which is one of my clearest memories, and if done properly the aircraft shouldn’t have moved!
If you want to start another hare, the Hunter was known for its “blue note", particularly on its run in to the break; the Canadians we exchanged with called it the “Annie Laurie”; we had F.4s, and there were reputedly differences with the Avon 200 powered later marks. These were after my limited time though.    Mike

Any single-seater driver would tell you – even in the ‘fifties – that size wasn’t important. And Just don’t talk to any Javelin crew about spin recovery.