I suspect you're talking about "pointer to member", which doesn't exist in C; it is a C++ concept. This is not a pointer, but a kind of indirection mechanism dressed up in pointer syntax.

I've never seen C++ code which depended on the internals of pointers-to-member, or assumed they had the same size as other kinds of pointers.

A pointer to member is actually a complicated offset into an offset into some structure associated with an object's class. This offset can be "dereferenced" with respect to objects of different types in the same hierarchy, taking into account multiple inheritance and virtual bases, etc. A pointer to a Base::Foo function, can be applied against a Derived object, such that it resolves to the correct code, even though Base is the third base of Derived, and so Foo is in a totally different vtable position in Derived compared to Foo. (The pointer cannot be a blind integer offset.)

In C, we would make a struct containing function pointers, and a simple "pointer to member" would just be an integer "offsetof(our_struct_type, some_function)". Then worry about it later if someone wanted multiple inheritance. :)

Nevertheless, it is a kind of pointer (it's called a "pointer") with a non-uniform size. That its internal structure is composite (just like a segmented 80286 far pointer) is irrelevant.