Reproduction steps:
1. Open attached project;
2. Click on the object "CapsuleCollider2D_bugged" and observe auto calculated mass value;
3. Click on the object "PolygonCollider2D" and observe auto calculated mass value;

Actual result: the mass of the Capsule Collider 2D is calculated wrong (in this case, it is ~1.285).
Expected result: the mass of the object Capsule Collider 2D should be calculated right (in this case, it should be ~1.785).

Reproduced with: 5.5.0a1, 5.5.2p3, 5.6.0f1, 2017.1.0a3

Can not be tested with older versions than 5.5 because there is not a component "Capsule Collider 2D".