Before taking any dangerous, time-consuming or strategic actions, you should establish exactly what in your program is using up so much of the memory. You may think you know the answer, but until you have evidence in front of you, you don't. There's the possibility that memory is being used by something you weren't expecting.
Use a profiler. It doesn't matter which one, there are plenty of them. First find out how much memory is being used up by each object. Second, step though iterations of your serializer, compare memory snapshots and see what objects or data are created.
The answer will most likely be to stream the output rather than building it in memory. But get evidence first.
