It does not use more RAM, these are references to structs that already exist inside instances. That’s how GML works now.
You can find these two lines in GMRoomPack’s GML file
var l_list = room_pack_raw_store_instances;
if (l_list != undefined) ds_list_add(l_list, self);
and change the second line to use id instead of self, but I do not think you’ll win anything whatsoever in doing so.