The memory saving comes from converting:

    Py_ssize_t dk_size;
    dict_lookup_func dk_lookup;

to:

   uint8_t dk_log2_size;
   uint8_t dk_loopup_kind; /* Only 3 possible values */
   uint32_t dk_version;

which saves 8 bytes on a 64 bit machine (no saving on a 32 bit machine).