First of all, the OS is only about 650mb, not 2.5GB
Second, OP, this question has been asked many times on this forum and their have been solutions detailed on how to fix this, its a little late for me and I don't want to go page searching, but they're here (somewhere).
I myself have had this issue with both my iPhone and my iPad, and I have documented this issue on video (screen capture) and I asked the Genius what they thought could be the problem, but they (4 different Genius's) couldn't explain what was going on.
The one solution that someone on this forum suggested that actually worked for me was to:
-Plug in your iPhone to iTunes and before you begin to sync, add an album or movie to your iPhone
-Then hit Add/Sync
-Now when the progress bar gets to the point that its going to add the new song/movie (image above), yank the USB cable from the iPhone
-Your iTunes will freeze for a moment (sometimes even minutes), but once the status bar returns to the Apple logo, plug your iPhone back in and the 'Other' data should have re-set back to what it should be.
Edit:
it actually wasn't to hard to find.
https://forums.macrumors.com/threads/1081908/