All things being equal, you have to throw the small diameter disc harder. It needs to get up to a higher speed to do what it does, namely penetrate forward. The larger diameter disc is going to glide and can do that at a lower speed. However, in reality it ends up being apples and oranges becasue stability also effects how hard you need to throw. So in reality you throw a large-diameter overstable disc harder than an understable small diameter disc. Once you are on the course there are so many other factors in play that it's never as simple as "small diameter, I need to throw it hard."
I don't have a lot of experience with small diameter mids. I really don't like them at all. From the experience that I do have (mostly throwing Panthers and MRV's) the small diameter discs work well on a low line while the large diameter discs do better with some height. This might be fubar as I really don't have a lot of throws with the small diameter mids, and the few throws I do have didn't get me excited.