I investigated your problem with the RealSense Viewer and experienced the same (files are saved to the machine's default documents folder). The only solution I can think of is to move the bag file to another folder or drive each time that you create one instead of doing multiple recordings and letting the storage space used increase to a large amount. I would not recommend changing your default document folder location.
The RealSense SDK has an Issues page on Github that is monitored by the RealSense engineering and development teams. You can post problems here, and if the message is not answered in a timely manner then it is automatically passed to Intel management. The idea is that everyone who posts an issue should get a reply eventually.
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