Huge heap dump file, but profiler states heap is small

So I have a 20 GB heap dump file.

When I open it up with jhat and Memory Analyzer I see that the overall size of all objects is appoximately 2.3Gb.

So, naturally the question is, why? The heap cannot be empty, it was running out of memory.

Is the serialized size of the heap dramatically different from the size when it is actually in use in the JVM?

I also opened it up with a hex editor, and I do not see huge portions of i开发者_如何学Pythont being empty.