Character's head flattens, very weird deformation

So I am doing a project with a homemade model. When I import it as an asset into Unity, its head flattens, but the rest of the model is normal, which is very weird, to say the least! Have you encountered anything similar before? If I do a preview of the components of the asset, the mesh itself looks normal in the preview window, but the previewGRP object looks deformed in the same way as the entire asset. Pictures: https://imgur.com/a/Z5BCy

 

This issue has been solved! For posterity, here is the solution:
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If you select your model in Unity and change your Rig mode to "none", the model will be normal and also be smaller. This means when you rig your model and do your animation, your joints are scaled in a wrong way. You can check your X scale value (probably it's 0 that's why it is smashed) Unfortunately, if you want to use your animation, you need to go back to your rig and rescale it
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The reason i didn't notice that the scale value was wrong, is that if I selected ALL joints it would say that the X scale value is equal to 1. I had to select the SPECIFIC joint which was wrong.