If NXDumptool determines that it needs to split it will create xxx.00, xxx.01, etc., etc. there won’t be any hidden files. They will all be in a folder (that will look normal under windows) that simply has the archive but set.
How are you determining space used? Are you sure you are using the same metric? GB != gb
E: I’m dumping my Super Mario Odyssey right now and will walk you through it:
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In this image it’s telling me it;s writing the 00 file.
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In this one we have moved on to the 01 file.