In my experience, 2-3 gb of OTA update file was downloaded and it required me 5.4 GB of free space to extract and install.
Could be, my point... anyone who have iTunes on a PC/Mac can avoid that hassle because it only needs that space on the phone for extracting when updating over the air. I can imagine being pain for anyone with 16GB of storage, something I did quit buying over a year ago.
I have never come across any OTA update bigger then 1.5gb but then it may very from one model to another.