What you want to do is called pattern grading. We don’t have a tutorial on this yet but you can find an article about it here: http://www.threadsmagazine.com/item/4368/making-sense-of-pattern-grading/page/all Doing something like a men’s vest should be easier than their examples so you might be able to just add your inches like you suggest. If you do this, I do encourage that you buy some cheap fabric, like muslin and baste your pieces together, so you can test the fit and see if you need to make any adjustments to your pattern. Good luck! 🙂