Often distortions can come as a result of modifiers and/or scales existing through your transform hierarchy. In Object mode in Blender, always have your rotations set as (0,0,0) and your scales set as (1,1,1) otherwise you will have issues. You can apply those transforms in Blender.
Unity runs this script to import your Blender files via transient FBX export:
./Hub/Editor/2020.1.5f1/Unity.app/Contents/Tools/Unity-BlenderToFBX.py
./Hub/Editor/2020.2.1f1/Unity.app/Contents/Tools/Unity-BlenderToFBX.py
./Hub/Editor/2019.4.10f1/Unity.app/Contents/Tools/Unity-BlenderToFBX.py
./Hub/Editor/2019.4.8f1/Unity.app/Contents/Tools/Unity-BlenderToFBX.py
If you’re still having trouble, your choices are:
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study and modify the above script
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export the FBX yourself and only have Unity import the FBX, which gives you more confident control.