The Redmond company confirmed the news in an email to tech website Softpedia, and put the figure down to the presence of the Windows 8 operating system and the pre-installed applications on the tablet.
Microsoft stressed that Surface Pro users will be able to save files via SkyDrive (which gives 7GB of free cloud storage space) and the USB 3.0 port, but the news could mean that the entry-level 64GB Surface Pro comes with just 19GB of available storage.
This isn't the first time Microsoft has had to contend with storage problems on the Surface tablets.
Back in November, the company confirmed that the 32GB and 64GB Surface RT tablets would come with around half of the advertised storage, with this again owing to Windows, Office and the pre-loaded applications.
Microsoft announced last week that it intends to launch the Surface Pro family on February 9. The range starts at $899 and accessories for the Intel-based tablets are expected to land on the same day.