This style of small string optimization tries to take up the same amount of space on the stack as a "normal" heap-allocated string. On 64-bit platforms that is 24 bytes: 8 bytes for the pointer to the heap allocation, 8 bytes for the number of characters/bytes in the string, and 8 bytes for the allocation capacity.

It's quite possible to make the small string buffer larger, but that comes at the cost of the large string representation taking up more space than necessary on the stack. IIRC libstdc++ does this, which makes its std::string take up 32 bytes on the stack.