Understanding Node.js Resident Set Size vs. Heap Size (OOM errors)

I'm fighting against a problem in my node server where I get an error that causes the node app to crash:

FATAL ERROR: JS Allocation failed - process out of memory

I'm using nodetime to take a look at the memory usage. I think perhaps I'm narrowing down on the problem, but I'm still pretty confused. Check out this function, which uses Mongoose to load a cached object from MongoDB:

StreamCache.prototype.loadCachedStream = function(_id, callback)
{
    this.model.findOne({'_id': _id}, {'objects':1,'last_updated':1}, function(err, d){
        callback(err, d ? d.toObject() : null);
        //The toObject() seems to cause the RSS to move into heap...?
    });
};

Notice the commented line. Prior to 11pm last night, the line was just

callback(err,d);

I added the toObject() call at 11pm last night.

Now look at my memory charts:

Notice that prior to this change, the RSS grew but not the heap. After the change, the heap and RSS grew exactly the same (until the app crashed). Note that the out of memory error (above) was happening both before and after the change. However, the change seems to have made the heap size correlate in its leaks to the RSS size, where before the heap was flat(ish).

My assumption is that, for some reason, this means the toObject() function moved the leaked data from RSS into the heap, so not only the RSS was leaking but also the heap.

Does that sound right?

If so... any ideas what might be causing the issue?