Why does mmap file use more memory than file size?

I am experimenting with mmapand came up with the following code example:

    int main() {

    int fd;
    char *filename = "/home/manu/file";
    struct stat statbuf;
    int i = 0;
    char c = *(filename);

    // Get file descriptor and file length
    fd = open(filename, O_RDONLY);
    if (fd == -1) {
        perror("fopen error");
    }
    if (fstat(fd, &statbuf) < 0) {
        perror("fstat error");
    }
    printf("File size is %ld\n", statbuf.st_size);

    // Map the file
    char* mmapA = (char*) mmap(NULL, statbuf.st_size, PROT_READ, MAP_PRIVATE,
            fd, 0);
    if (mmapA == MAP_FAILED) {
        perror("mmap error");
        return 1;
    }

    // Touch all the mapped pages
    while (i < statbuf.st_size) {
        c = mmapA[i];
        i++;
    }
    c++;

    // Close file descriptor
    if (close(fd) == -1) {
        perror("close");
        return 1;
    }

    //Unmap file
    munmap(mmapA, statbuf.st_size);

    return EXIT_SUCCESS;
}

File size: 137948 bytes = 134.7 kilobytes. To check the program memory, I use the upper, mainly RES and VIRT columns. I look for these values ​​in three different places:

• before calling mmap
• right after the call mmap
• after reading all the associated memory so that the file is loaded into main memory (after page errors)

The value shown above

• VIRT = 1828 RES = 244
• VIRT = 1964 RES = 248
• VIRT = 1964 RES = 508

1964 - 1828 = 136, I think, in kilobytes and, thus, perfectly fits the file size.

But I can not understand the difference RES 508 - 248 = 260 .. Why is it different from the size of virtual memory and file size?