When retargeting large characters, you might find the pose is deformed when the Retarget IK setting is enabled.

This is a result of the way the Full Body IK solver works and can easily be fixed.

First, if you don't intend for your character to be so large, then the best option is to check the import/export settings of the model to fix the scale.

If you do intend on using giant characters, then this can be fixed by opening the IK Rig asset generated by the plugin, selecting the Full Body IK in the Solver Stack, and increasing the Min Mass Multiplier value.

The Unreal documentation recommends keeping this value as low as possible, so a good way to tune this is to grab and move one of the IK Goals then gradually increase the value until you get a stable result.