Memory leak when read file in C++?

here is my code:

// the read func
vector<string> read(string file_name){
    ifstream ifs(file_name, ifstream::in);
    string line;
    vector<string> company_names;
    while(getline(ifs, line)){
        company_names.push_back(line);
    }
    return company_names;
}


void test(){
    vector<string> company = read("/home/yanwii/SocialCredits/CompanyName/company_names.txt");
    vector<string>().swap(company);
}

int main(){
    test();
}

After loading from file, in htop the program occupied 2G, then After swapping, the program still occupied 1G, what's going wrong?