In C#/CLR reference types have some size overhead:

The layout of a managed object is pretty simple: a managed object contains instance data, a pointer to a meta-data (a.k.a. method table pointer) and a bag of internal information also known as an object header.

So when you box your int into the heap it will be stored as a reference type and will include all the extra info and then the memory will be aligned resulting in something like this:

Object Header (8 bytes)
Method Table Pointer (8 bytes)
Int (4 bytes)
padding (4 bytes, to align at 8 bytes)

Note that this is dependent on the bitness of the process (the breakdown above is for 64-bit case) and is actually runtime implementation detail (the previously linked article mentions at least one implementation which differs).