What determines object size here?

I have created simple object containing integer and few methods, integer primitive variable alone and compare their sizes. "sizeof()" for both returned value of "4". Why is that - shouldn't object that is composite type and contains information about methods take more space?

#include <iostream>

class Person{

private:
int a;

public:

void hello(){
std::cout << "hello";
}

void DoSomething(){
    a++;
}
};

int main(){

int a;
Person p;

std::cout << sizeof(a) << std::endl;

std::cout << sizeof(p) << std::endl;

return 0;
}