"It seems obvious, so I don't know why it took me so long for the penny to drop, but the trick to better mocap retargeting is matching your target armature's hip placement and limb proportions to the source before matching the rest of the pose. The slightest difference between the relative position of the hip to legs and spine can create serious issues with limbs sliding or bouncing that stop a character from feeling rooted.

You can minimise issues by first scaling the source armature to match the target character, but that only gets you so far. Lining up the hip bones is the best value adjustment you can make, but for ultimate accuracy, limbs need a similar treatment. Finessing joint positions to line up can improve things, but it risks messing with the character's deformations. Scaling limbs is the next best thing – at the cost of some stretching/squashing, you can line up joints more closely while maintaining the original joint positions."