There’s one important thing that you must do to ensure that your model exports correctly.
In blender, select all objects in the scene (in object mode, press A and check that all of them are lit with the orange outline) then press
CONTROL A
and then
APPLY POSITION
APPLY ROTATION
APPLY SCALE
then export again.
Usually, deformation occurs because of uneven scale between objects.
Additionally, you need to be sure that object dimension is ‘realistic’. As a matter of fact, the original blender cube of a new scene, is a 2meter by 2meter wide object, that will be the same size in unity!
So make sure that your objects dimensions are in the units rather than in the hundreds of units.