I don't think 7mp non-massive/non-mobile Large-figure movement is strictly better than 4mp for small Mobile figure. The Dewbacks can move in straight lines, but they are less agile when running corners.
It's pretty rare when the Dewback's more MP won't be better than the doid's flexible movement. The 3 alone will often be enough to get it into position to do the shock lance and attack, and when it isn't, 7 is enough to take a shot and get into cover. The size is also good for blocking LoS or just making the wall of flesh to prevent enemy movement.