Why does mongodb virtual memory get so large
MongoDB uses memory mapped files for performance I'll point you to a longer answer from me on this here but in essence large amounts of virtual memory usage are not something to worry about. In the case of virtual memory you'll see double usage with journaling, however this is less drastic then it sounds as the space is not actually used until there is an update. It is marked to the OS as being used despite being potentially empty when you start. 

These aspects are all elements of how MongoDB uses the memory subsystem to provide performance, similarly to how it preallocates disk space to ensure these resources are marked and ready to serve your connections when they occur.

There's an interest presentation on MongoDB's storage layer that explains how the storage is managed and allocated, aggressively in most cases which tends to bump up these numbers.