Problem in defining of Vector class Copy Constructor (C++)

I tried to define a Vector class and some functions of it manually. Everything seemed like okay, until I wrote the copy constructor, where I get a warning in this part of the loop:

this->arr[i] = vec.arr[i];
C6386:Buffer overrun while writing to 'this->arr'

#include <iostream>
using namespace std;

class Vector {
private:
    int size, capacity;
    int* arr;

public:
    Vector(int _capacity = 5) : capacity(_capacity) {
        size = 0;
        arr = new int[capacity];
        cout << "constructor" << endl;
    }
    ~Vector() {
        delete[] arr;
        cout << "destructor" << endl;
    }
    Vector(const Vector& vec) {
        this->capacity = vec.capacity;
        this->size = vec.size;
        this->arr = new int[this->capacity];
        for (int i = 0; i < size; ++i) {
            this->arr[i] = vec.arr[i]; //WARNING. C6386:Buffer overrun while                     writing to 'this->arr'
        }
        cout << "copy constructor" << endl;
    }
    void push_back(int n) {
        if (size==capacity) {
            resize();
        }
        arr[size] = n;
        size++;

    }
    void resize() {
        int* tmp = new int[capacity*=2];
        for (int i = 0; i < size; ++i) {
            tmp[i] = arr[i];
        }
        delete[] arr;
        arr = tmp;
    }
    int Size() { return size; }
};


int main() {
    Vector V1(3);
    V1.push_back(1);
    
    cout << V1.Size();
    V1.push_back(4);
    cout << V1.Size() << endl;
    V1.push_back(4);
    cout << V1.Size() << endl;
    V1.push_back(4);
    cout << V1.Size() << endl;

    Vector V2(V1);

    return 0;
}

I thought the problem is that I'm not careful enough and I maybe let the loop get out of the bounds somewhere, but it seems like that is okay.

Can you please help me to define the problem?