C heap-allocated array undefined behavior

Question

I'm dynamically allocating an array in main() to store the prime factors of any number calculated through a PrimeFactors() function I coded myself. Problem is, when the size of the array >= 4, the program displays what looks like undefined behavior; reading garbage values and whatnot. Works perfectly when size < 4.

I thought that maybe the array address was changing through the function and as a consequence main() would read garbage values, not knowing the "new" address of the array; and it seems like this is the case.

OS is Linux Ubuntu 18.04 LTS.

As i am learning I do not want to simply use vector and forget about this problem.

Why is this happening and what could I do to fix this ? I am clueless and it seems google and SO are as well.

Here's the main() code :

int main(int argc, char* argv[])
{
    unsigned long long n = atoll(argv[1]);

    unsigned long long* factors = malloc(sizeof(unsigned long long));
    if(factors == NULL)
    {
        printf("malloc failed to allocate factors\n");
        return -1;
    }
    else
        printf("First address in main() %p\n\n", (void *)&(factors[0]));

    unsigned short int* size = malloc(sizeof(unsigned short int));
    if(size == NULL)
    {
        printf("malloc failed to allocate size\n");
        return -1;
    }

    PrimeFactors(factors, size, n);

    printf("Last address in main() : %p\n", (void *)&(factors[0]));

    for(int i = 0; i < *size; ++i)
        printf("%llu\n", factors[i]);

    free(factors);
    free(size);
    return 0;
}

And here's the PrimeFactors() code :

int PrimeFactors (unsigned long long* factors, unsigned short int* size, unsigned long long n)
{
    printf("In PrimeFactors() :\n");
    *size = 2;
    factors = realloc(factors, (*size) * sizeof(unsigned long long));
    if (factors == NULL)
    {
        printf("realloc failed re-allocating factors array\n");
        return -1;
    }
    else
        printf("realloc() %d address in PrimeFactors() : %p\n", *size, (void *)&(factors[0]));

    if (IsPrime(n))
    {
        factors[0] = n;
        return 0;
    }

    unsigned short int factorsCount = 0;
    for (unsigned long long i = 2; i <= n; ++i)
    {
        while(n % i == 0)
        {
            if(factorsCount >= *size)
            {
                factors = realloc(factors, ++(*size) * sizeof(unsigned long long));
                if (factors == NULL)
                {
                    printf("realloc failed re-allocating factors array\n");
                    return -1;
                }
                else
                    printf("realloc() %d address : %p\n", *size, (void *)&(factors[0]));
            }
            factors[factorsCount] = i;
            ++factorsCount;

            n /= i;
        }
    }

    printf("last address in PrimeFactors() : %p\n", (void *)&(factors[0]));
    for(int i = 0; i < *size; ++i)
        printf("%llu\n", factors[i]);

    printf("Exiting PrimeFactors()...\n\n");
    return 0;
}

Compile flags :

gcc -Wall -Wshadow -Wpointer-arith main.c libMath.c -o learnlab.exe

Execute command :

./learnlab 80

Expected output :

First address in main() 0x5626aa180260

In PrimeFactors() :

realloc() 2 address in PrimeFactors() : 0x5626aa180260

realloc() 3 address : 0x5626aa180260

realloc() 4 address : 0x5626aa180260

realloc() 5 address : 0x5626aa180260

last address in PrimeFactors() : 0x5626aa180260

2

2

2

2

5

Exiting PrimeFactors()...

Last address in main() : 0x5626aa180260

2

2

2

2

5

Actual output :

First address in main() 0x5626aa180260

In PrimeFactors() :

realloc() 2 address in PrimeFactors() : 0x5626aa180260

realloc() 3 address : 0x5626aa180260

realloc() 4 address : 0x5626aa1812b0

realloc() 5 address : 0x5626aa1812b0

last address in PrimeFactors() : 0x5626aa1812b0

2

2

2

2

5

Exiting PrimeFactors()...

Last address in main() : 0x5626aa180260

0

2

2

4113

7233098161058900294

Sorry this is a long post, I couldn't really figure out a way to make it shorter. Thank you.