*Plaits beard in speculation*
Yo dude, judging by your post, I don't think you have many problems in essay writing. If you understand the topic more than the others, that's what'll get you the best marks, after all, for Extension History (as I understand it), your Major work needs to be of a certain word limit, so if you can get in 5 pages what it takes someone else to do in 9 pages, you have a definite advantage and you'll end up fitting in more for your Work.
I think you're mistaking that quantity is better than quality. If I were to write 10 pages and you 5 it's very possible that you'd getter better marks, as your answer would probably be more succint and to the point, (unless you didn't finish or only put in one or two points).
Generally, if you are meant to write an essay in a given time (this is probably more for English), you should have a few points on what your texts, or your ideas, quotes, etc. If it's a hand in task, try and write down first everything you want to say, and then scaffold it into a reasonable order, adding quotes, extra information under each idea. Then reread it, getting rid of things that are irrelevant or pointless, focusing more on the main ideas.
And my advice: Don't ever write for length. You'll end up putting in things that will make your marker cringe because they are not needed in the question. Never ever ever ever ever
says Santa Claus. It should be you that's happy - you understand the subject more than those 9 pager-people. And 5 pages by the way is still a hell of alot to write!
Good luck next year!