unique_ptr size is twice bigger than `void*` or `std::unique_ptr`

[X547]

std::unique_ptr<Object> ptr1;
unique_ptr<Object> ptr2;

printf("sizeof(ptr1): % " B_PRIuSIZE "\n", sizeof(ptr1)); // 8
printf("sizeof(ptr2): % " B_PRIuSIZE "\n", sizeof(ptr2)); // 16