Problem with free() on structs in C. It doesn't reduce memory usage

I'm having a problem with free() on a struct in my C program. When I look at /proc//statm before and after the free it doesn't seem to reduce. Am I using free() wrong in this case, or am I reading /proc//statm wrong?

Here is a test case which yields the problem:

struct mystruct {
    unsigned int arr[10000];
};

void mem() {
    char buf[30];
    snprintf(buf, 30, "/proc/%u/statm", (unsigned)getpid());
    FILE* pf = fopen(buf, "r");
    if (pf) {
        unsigned size; //       total program size
        unsigned resident;//   resident set size
        unsigned share;//      shared pages
        unsigned text;//       text (code)
        unsigned lib;//        library
        unsigned data;//       data/stack
        unsigned dt;//         dirty pages (unused in Linux 2.6)
        fscanf(pf, "%u %u %u %u %u %u", &size, &resident, &share, &text, &lib,         &data);
        printf("Memory usage: Data = %d\n", data*sysconf(_SC_PAGESIZE));
    }
    fclose(pf);
}

int main(int argc, char **argv) {
    mem();
    struct mystruct *foo = (struct mystruct *)malloc(sizeof(struct mystruct));
    mem();
    free(foo);
    mem();
}

The output is:

Memory usage: Data = 278528
Memory usage: Data = 282624
Memory usage: Data = 282624

When I would expect it to be:

Memory usage: Data = 278528
Memory usage: Data = 282624
Memory usage: Data = 278528

I've done a similar test with malloc'ing a (char *), then free'ing it and it works fine. Is there something special about structs?