Steps to reproduce:

1. Open attached project
2. Open scene "AreaEffectorTest"
3. In the scene there are two squares. Left one has four box colliders and right one has one
4. In hierarchy, select "AntiGravityAreaEffector" and notice that Force Target is Rigidbody
5. Run the scene and notice that left one shoots up and the right one does not. This happens because left one had the force of the effector multiplied by the number of colliders it has

Notes:
- Colliders of the left cube are resized so that their combined surface area would be equal to the surface area of the right one.
- This happens no matter if Rigidbody2D has Auto Mass enabled.
- If we were to set mass of the left square to 4 while keeping mass of the right one equal to 1 then both cubes would be affected in a same way.
- This issue doesn't happen with other effectors - only area and point are bugged

Reproduced with: 5.2.4f1, 5.3.4f1, 5.4.0b11