I’ve noticed Retargeting work best if before retargeting you do change two things:
i) the preview mesh
ii) the pose you use for the retargeting; metahuman come with a variety of these to cover all the various combinations (same as for the preview mesh: female vs male, normal slim overweight, normal tall short)
This is very well explained in the official documentation: Retargeting Animations onto a MetaHuman | MetaHuman Creator Documentation