If I compile the following with g++ -g a.cpp I get an executable of size 100,100 bytes. Ok.

// content of a.cpp
#include <string>

std::string  m_name_a;

int main() { return 0; }

But when I add another file g++ -g a.cpp b.cpp:

// content of b.cpp
#include <string>

std::string  m_name_b;

The executable size is 135,00 bytes.
Why an increase of 35%?