I lost 2.3 GB after unjailbreak my iPhone 4s I just unjailbreak my iPhone 4s to the original 5.1.1 version. 
I notice the devise is now SUPER slow and also I noticed that the storqage capacity lost 2.3 GB !! 

I have 16 GB iPhone 4s and iTune showing [Capacity 13.62 (.31 other) (13.4 free)]

The way I jaibreaked my iPhone is by 
- downloading 5.1.1 from iTune 
- enter the DFU mode, (power button and home button for 10 sec then only home button for 8 sec)
- click restore 

How can I unjailbreak my devise in the correct way? (if I did it wrongly)
What is the reason for losing the storage?
Whether you're JailBroken or not, and regardless of if you've ever been, you'll never actually see the full 16GB on your iPhone. Part of that 0.31GB of "Other" that iTunes reports are the remnants of the JailBreak. Restore iOS again, and then "Set up as a new phone" when going through the activation process. You'll never gain back that 2.3GB, because that's what iOS is using to store the operating system. To be more detailed, the 13.62GB of capacity is only the capacity of the User's partition on /var/mobile/. The other 2.3GB is given to / (the System partition).