To reproduce:

1. Open Project
2. Set Sprite's MeshType to FullRect
3. Drag sprite into scene
4. Add PolygonCollider2D
5. Notice the PolygonCollider2D mesh will be a box mesh instead of a tightly packed mesh.
6. Open Sprite Editor
7. Open 'Edit Physics Shape', notice how mesh is tightly packed
8. Click cog on Polygon Collider 2D and click reset
9. Notice the PolygonCollider2D now uses the correct Tightly packed physics mesh.

Reproduced: 2017.2.1p1, 2017.3.0p1, 2018.1.0b2, 2018.2.0a1
Not reproducible: 5.6.5f1, 2017.1.2p4, 2017.2.0b10