Don't think it has anything to do with size, if the correct size seat-post is fitted to frame there is barely any flex required.
If it was to give differing flex as diameters change, why does the red 34.9 above and my 31.8 both have the same wider unmachined bit?
The latest ones by Hope have the groove all the way round so how does that work? Why do the black 31.8 above and my 31.8 have different sized unmachined bits if they supposedly have to flex the same amount? You could just not machine the groove at all but make the band part of clamp thinner to give same flex. There are examples of other makes of QR clamps designed like that. As I said above Hope has just evolved the design, aesthetics, cost, strength and ease of manufacture will all have changed over the years.