Question RigidBody struct size & layout

Code (CSharp):

1. /// <summary>   An instance of a collider in a physics world. </summary>
2. /// Using StructLayout to handle for an issue where there can be a mismatch between struct sizes for RigidBody in separate jobs
3. [System.Runtime.InteropServices.StructLayout(System.Runtime.InteropServices.LayoutKind.Explicit, Pack = 1, Size = 56)]
4. public struct RigidBody : ICollidable, IAspectQueryable
5. {
6.     ...
7. }

I found this in RigidBody.cs. It scared me a little, while at the same time piquing my curiosity.

What could cause this kind of struct size mismatch?