Hello, thanks for reaching out about this retargeting issue.
Erratic root motion scale can be caused when the bone considered as the “root” of the body (this is separate from the root of the skeleton) is on or near the floor.
The solution is to set the retarget root to the Pelvis/Hips bone. And use a Pin Bone Op in the post settings to generate the motion for the skeleton root (“Root” bone).
Please let me know if that helps.
EDIT: Use a Root Motion Generator Op, not a Pin Bone. You could use Pin Bone to pin the root to the other root bone, but the RMG op gives you more control.