In C++, sizeof(T) only gives the size of the type T itself, including the size of its members and pointers, but it does not account for dynamically allocated memory or resources managed outside the object (such as heap-allocated memory pointed to by pointers). Therefore if you want to know how much memory is being used up by an object you cannot rely on sizeof
Note that if an object does not use an any dynamically allocoated memory then sizeof returns what you expect, otherwise it will require you to manually deduce the memory footprint of that object. This explains why you can have a c style array and sizeof will return the actualy memory footprint but the same on vector will not.
Specifically the equation for the memory footprint accounting for both memory stored on the stack and heap is sizeof(T) + size_of_owned_memory_on_heap.