Im learning to export .anim to FBX animation

I tried to apply an animator on the model and play the .anim file. Works good

I tried to export model+animation use FBX exporter and read in maya or else 3d workstation. Works good.

I tried to export animaion only. The bones just get stretched to weird scale.

I dont understand what would cause this happen.Anything I can do to avoid this?