To make sure the rider doesn’t lose/gain size you have to adjust the associate socket scale in the dino skeleton. Doing this at this current time has either crashed servers, or not transferred the changes made. For your experimentation, go into your dino’s skeleton, under the bone “back3” (i think) should be the “rider socket”. You can adjust that to change the scale of the rider when on the dino.
Let me know if you can get that working on a server, at this current moment I am not able to, but as soon as I figure it out, I will let you know.