Saving of 2-D array to the binary file in C language

I'm trying to save a 2-D array of integers to a binary file. The problem is that only the first row of the 2-D array is ​​saved. Below I have attached the outputs from which you will better understand the problem. Thanks in advance for the help.

#include <stdio.h>
#include <stdlib.h>

#define N_MAX 100

// Declaration of functions
int readMatrix(FILE *fp, int matrix[N_MAX][N_MAX]);
void printMatrix(int matrix[N_MAX][N_MAX], int size);
void switchMax(int matrix[N_MAX][N_MAX], int size);
int avg(int matrix[N_MAX][N_MAX], int size);
int sumDown(int matrix[N_MAX][N_MAX], int size);
int sumUp(int matrix[N_MAX][N_MAX], int size);
void sortMainDiagonal(int matrix[N_MAX][N_MAX], int size);

int main()
{
    int size, matrix[N_MAX][N_MAX], test[N_MAX][N_MAX];
    FILE *fp;
    fp = fopen("input.txt", "r");

    if (fp != NULL)
    {
        size = readMatrix(fp, matrix);
        fclose(fp);

        switchMax(matrix, size);
        avg(matrix, size);
        sortMainDiagonal(matrix, size);

        printf("\nOriginal matrix: \n");
        printMatrix(matrix, size);
        
        // SAVE ON BINARY FILE
        fp = fopen("output.bin", "w");
        if (fp != NULL)
        {
            fwrite(matrix, sizeof(int), size*size, fp);
            fclose(fp);
        } else {
            printf("Error #e2");
        }

        // READ FROM BINARY FILE
        fp = fopen("output.bin", "r");
        if (fp != NULL)
        {
            fread(test, sizeof(int), size*size, fp);
            fclose(fp);

            printf("\nMatrix from file: \n");
            printMatrix(test, size);
        } else {
            printf("Error #e3");
        }
    } else {
        printf("Errore #e1");
    }
}

// Definition of functions
int readMatrix(FILE *fp, int matrix[N_MAX][N_MAX])
{
    int size = 0;
    fscanf(fp, "%d", &size);
        for (int i = 0; i < size; i++)
            for (int j = 0; j < size; j++)
                fscanf(fp, "%d", &matrix[i][j]);
    
    return size;
}

void printMatrix(int matrix[N_MAX][N_MAX], int size)
{
    for (int i = 0; i < size; i++)
    {
        for (int j = 0; j < size; j++)
        {
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }
}

void switchMax(int matrix[N_MAX][N_MAX], int size)
{
    int max = 0, row = 0;
    for (int i = 0; i < size; i++)
    {
        for (int j = 0; j < size; j++)
        {
            if (matrix[j][i] > max)
            {
                max = matrix[j][i];
                row = j;
            }
        }
        matrix[row][i] = matrix[i][i];
        matrix[i][i] = max;
        max = 0;
    }
}

int avg(int matrix[N_MAX][N_MAX], int size)
{
    int avg;
    avg = (sumDown(matrix, size) + sumUp(matrix, size)) / 2;
    matrix[size-1][size-1] = avg;
    return avg;
}

int sumDown(int matrix[N_MAX][N_MAX], int size)
{
    int sum = 0;
    for (int i = 1; i < size; i++)
        for (int j = 0; j < i; j++)
            sum += matrix[i][j];

    return sum;
}

int sumUp(int matrix[N_MAX][N_MAX], int size)
{
    int sum = 0;
    for (int i = 0; i < size-1; i++)
        for (int j = i+1; j < size; j++)
            sum += matrix[i][j];

    return sum;
}

void sortMainDiagonal(int matrix[N_MAX][N_MAX], int size)
{
    int temp;
    for (int i = 0; i < size-1; i++)
    {
        for (int j = 0; j < (size-1-i); j++)
        {
            if (matrix[j][j] > matrix[j+1][j+1])
            {
                temp = matrix[j][j];
                matrix[j][j] = matrix[j+1][j+1];
                matrix[j+1][j+1] = temp;
            }
        }
    }
}

Screenshot of outputs from 'printMatrix' function: click

File 'input.txt':

4
5 6 1 8
1 20 3 4
9 0 11 12
13 4 15 1