Yes it does dear Reader. In many ways.
First of all, if you look at postage rates, you are charged more for large sizes, same weight, than for small sizes. Check with your PO to be sure you are getting the biggest bang for your buck.
From a Direct Mail perspective, you will make a larger impact with a larger peice. The trick is to go to the maximum size at the lowest rate.
A 6 x 9 envelope has 54 sq, inches of "real estate"; a #10 envelope has less than 40. That is definitely some extra room in which to state your case, front and back.
Let's face it, a lot of people, although they have to look at their mail, just look at the covers...they are judging your book by your cover. And deciding quickly - stay or go.
Take advantage of that; tease the offer, entice them with beauty, money, reward right on the cover. Get them to go inside; that's where the action is. Make that space work for you and grow your ROI.
Go big, or go home.