Not all devices get the latest updates available at the same time - plus with 1803 there have been various blocks put on the update for some hardware that have or will be released at different times i.e. Toshiba SSDs, etc.
https://winaero.com/blog/how-to-see-update-history-in-windows-10/ should show you if an update has been applied recently and/or the updates page in control panel will show you any that are in progress.
Well all my 5 devices had been updated to 1803 through Windows Update.
I looked in update history on my 8700K desktop, there was 3 quality updates, 1 driver updates and 1 other updates. Checked Microsoft Update Catalog for updates file size:
Quality Updates (3)
2018-05 Security Update for Adobe Flash Player for Windows 10 Version 1803 for x64-based Systems (KB4103729) -
20.9MB X64 & 10.2MB X86
2018-05 Cumulative Update for Windows 10 Version 1803 for x64-based Systems (KB4103721) -
326.5MB X64 & 148.6MB X86
Security Update for Microsoft Office 2010 (KB4022137) 32-Bit Edition -
17.4MB X64 64-Bit Edition & 8.2MB X86 32-Bit Edition
Driver Updates (1)
Intel Corporation - SoftwareComponent - 4/24/2018 12:00:00 AM - 2.0.101.44269 (Intel(R) Software Guard Extensions Platform Software Component)
2.7MB
Other Updates (1)
Windows Malicious Software Removal Tool x64 - May 2018 (KB890830)
60.1MB
My deskop running Windows 10 1803 X64 was downloaded 5 updates total size was 427.6MB so it was not a huge 9GB update after downloaded and installed 1803 from Windows Update with file size only 3.46GB for X64 and 2.62GB for X86.
It possible OP had huge 9GB update could be come from Microsoft Store updates for apps and games.