I noticed that Xcode doesn't show as installed in the Mac App Store after i deleted the space hogging 3GB installer from my Applications folder. So i installed Xcode again but grabbed the app from the private cache and then stripped out all the packages and then left the receipt, executable & edited on the icon. now its just a 540KB app which you can put in your Applications folder and Xcode will show as installed. I hid the app using this command in Terminal: sudo chflags hidden /Applications/Install\ Xcode.app
that's all good if you're nitpicky, but it's going to download that whole installer again to do updates. If you keep the installer you get delta updates. If you trash it you have to redownload.
You should get delta upgrades with this as the original 3GB installer just contained the install packages and the Mac App Store will see the version of this empty installer. Also, i think people aren't getting the recent delta update as it only corrected the installer and they already have Xcode installed. Backup the empty installer somewhere and put it back when it gets replaced by a new update installer. Plus you can put the empty installer anywhere on you hard disk and it will still show as installed in the MAS, so then just delete the big installer. It saves you 3-4GB of space, which is invaluable to SSD owners.