The size of each individual object is determined by calling malloc_usable_size on the block itself, so this includes any internal alignment, as well as any padd

The size of each individual object is determined by calling malloc_usable_size on the block itself, so this includes any internal alignment, as well as any padding from the allocator after the object proper. So, you get the size of the object, using the callback. Then, if the object has fields that point to other objects, you call the reporter on those fields. Then those two quantities are totaled.