Again, I guess I don't really understand your question. No reason to have a small bike on a gravel route. What's the point?
I rented a CRF250X and was completely disappointed by the top end. I had it sideways on a "gravel route" (forest service road) with it wide open. No joke, I wondered why I bothered with a 250 at that point. It was tall enough that it was hard to turn around on single track (for me, although I have a short inseam). My DR650 is no harder to turn around on single track because I can at least touch the ground where needed. I've never wished for more top end off road with 650cc and can seldom use it all (mostly due to the suspension of the DR650; although small 230 - 250 "dual sport" bikes don't have performance suspension either).