The issue in the AEC world is that unlike big database, Desktop CAD/BIM applications are relying on PC-level hardware to do what normally takes server arrays. CAD/BIM software is really, really nasty with disk access - with thousands of little read-write-open-lock-close commands on every element within the main BIM database. The example given - 1.5MB is TINY - a 'standard' highrise residential tower can run to 2-3GB file size. The open-lock-close file access requests can number in the millions for a file this size.
Do a test. Download the trial version of Revit from Autodesk, start up Sysinternals ProcMon and run a trace of the file open command on the basic sample file. You will be astonished at the amount of file access faffing around the software does. But hey ho, everyones been busy developing spreadsheets and Flappy Bird clones and not paying much attention to a massive field that is only responsible for, like, cities.