Spotfire SBDF size - why so big?

I have a data table with a lot of columns, but only four million rows.  If I save the DXP containing the data table with the data embedded, it is les than 200MB.  If I export the data table to an SBDF file, it is over 2GB.  If i save it to the library it is the latter size.  Why the big difference, and are there any options for reducing the storage size of data tables please?

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