Shell script help

How can I change my code to be able to compare the size of two files.

if [ -f $1 ] && [ -f $2 ] && [ ! $1 -ef $2 ]; then
    size1=$(du $1)
    size2=$(du $2)
    if [ $1 -gt $2 ]; then
     echo "$1 is bigger"
    else
      echo "$1 is smaller"
    fi
fi

/* ftell example : getting size of a file */
#include <stdio.h>

int main ()
{
  FILE * pFile;
  long size;

  pFile = fopen ("myfile.txt","rb");
  if (pFile==NULL) perror ("Error opening file");
  else
  {
    fseek (pFile, 0, SEEK_END);
    size=ftell (pFile);
    fclose (pFile);
    printf ("Size of myfile.txt: %ld bytes.\n",size);
  }
  return 0;
}