g++'s sizeof(std::string) is only 8 bytes because it's a single pointer such that the struct can be reinterpret_casted to a const char * and you get the same thing that c_str() gives you. It's reference-counted (across threads!), copy-on-write. I don't think people would mind a 24-byte std::string (on 64-bit systems), which would consist of a pointer, size, and capacity, and which would allow 22-byte strings to be stored inline.
On Windows, sizeof(std::string) is 24 or 32 depending on 32- or 64-bit. sizeof(vector<char>) is 12 and 24, respectively, of course. I believe the extra size of the std::string is for storing strings inline. I'd guess their implementation looks like struct { char *ptr; size_t size; union { size_t capacity; char buf[16]; } u; };, which sacrifices some size in favor of keeping the code paths simple.
On Windows, sizeof(std::string) is 24 or 32 depending on 32- or 64-bit. sizeof(vector<char>) is 12 and 24, respectively, of course. I believe the extra size of the std::string is for storing strings inline. I'd guess their implementation looks like struct { char *ptr; size_t size; union { size_t capacity; char buf[16]; } u; };, which sacrifices some size in favor of keeping the code paths simple.