Unfortunately it seems that this feature isn’t quite what it seems. On the surface it looked pretty straightforward where you’d press the “X” and the program would be removed. If you wanted the program again, you’d have to redownload it, kind of like what you would do with regular apps. However during John Gruber’s The Talk Show podcast, Apple’s SVP of software engineering Craig Federighi cleared up the misconception.
Apparently what happens is that this feature doesn’t actually remove the app from your device. Instead all it does is it deletes user data associated with the apps and hides the icon from view. The actual binary of the app will remain on your device. Now for those who are a little outraged since they thought they could free up some space, there won’t be much freeing up to begin with.
Federighi estimates that the combined total of Apple’s stock apps will probably take up less than 150MB of space, meaning that even if you have the 16GB iPhone, 150MB probably won’t be making that much of a dent.