NodeJS discrepancy between heapUsed and chrome inspector

Question

I am currently working on reducing the memory consumption of a NodeJS command line application. Using the chrome inspector I was able to locate a huge object that was being retained due to a coding error in the application.

After fixing the error I can no longer see the object (LargeObjectbelow) in the inspector and it seems the memory reduction effort was a success, however, I am puzzled by the following behaviour.

When executing nodejs using --expose-gc and observing the heap memory usage by running

<code that created an object 'LargeObject' containing very large Maps, ~400Mb)>
global.gc()
const used = process.memoryUsage().heapUsed / 1024 / 1024;
console.log(`The script uses approximately ${Math.round(used * 100) / 100} MB`);

I'm still seeing essentially the same number as prior to the bugfix. If I stop the program after the GC in the inspector there are no references to the LargeObject and the total memory under Retained Size is what I expect. (ie, reduced by around 400Mb)

If I however explicitly remove the references to the large Maps,

LargeObject.Map1 = null;
LargeObject.Map2 = null; // This is where LargeObject is in scope. It is *not* in scope in the above code

The process heapUsed print shows a drastically smaller number (dropped by around ~400Mb as I would expect)

I have quite a lot of trust in the chrome inspector and I feel confident that the object is no longer being retained, that said, I'd love to understand why the heapUsed value only drops in the second case. Is there a risk that something is still retaining the object but that the Chrome Inspector fails to detect it?