Ridiculous database size

I'm building a hash-based databased where key and data are both 8-byte
integers. There is roughly 50M entries, so the actual data size is
about 800M, but the produced db file is 2.1G. Why does it use so much
space for a hash table which is supposed to be a compact data
structure?

Here's my configuration:

        db.set_cachesize(10, 0 * 1024 * 1024, 1); //If I set it to 2G,
then it becomes terribly slow after 20M entries. I don't understand
why.
        u_int32_t pageSize = 4 * 1024; // block size on my machine
        db.set_pagesize(pageSize);
        db.set_h_ffactor((pageSize - 32) / (keySize + 8 + 8)); //
according to recommendation
        db.set_h_nelem(50000000);
        db.open(NULL, "test.db", NULL, DB_HASH, DB_CREATE |
DB_TRUNCATE, 0);

Thanks!