I don't know, but if it's possible it will require a lot of work because you have to:
1) Edit the 3D model to make it smaller and assign it to the desired rider
2) Adjust the bones to the bigger body
3) Make sure the animations with the bigger body do not get buggy
In UE4 you can retarget animations to any rig, so in theory Milestone could've had different body heights/sizes, just a feature looked past.