Does a bootable OS X drive need more free space than the installer?
I just made an installer image for OS X 10.10 to install in a virtual machine and it's only 5268873216 bytes or 4.90702 GB in Base2, in Base10 it's 5.27GB.  So you could have made it smaller.  Not sure if you can resize it, you can try and if not delete it and start with a smaller partition.  BTW There does need to a just a bit of free space as it's written to during the install.