Minimum Space for Boot Camp Windows 8.1
Have you changed your info.plist in boot camp assistant so you can use a .iso file for windows 8.1? 

If you havn't follow my steps!

Find Boot Camp Assistant then DUPLICATE IT put it on your desktop right click it show package contents open info.plist then find PreUSBSupportedModels and remove the Pre JUST THAT save and open! then download a ISO 

( Look up a activation key ) 

Here is download 
( Dont need key to download )

https://www.microsoft.com/en-au/software-download/windows8

Always STAY cool!

By the way this is just so you dont lose storage on your HD Harddrive