VF700/750 Rear Brake Disk???

Does the 85 VF700F have a smaller rear brake disk than the 84 VF750F? I ask because I purchased a rear disk from an 84 VF750 on eBay thinking it was the same size as the one on my 84 VF700 and when I got it in the one I purchased was about .5" larger. I've looked at a few parts diagrams online and as far as I can tell the only difference between the two rear brake systems is the disk itself. All the other parts such as the caliper, caliper bracket, and link arm all have the same part numbers, so why can't I get the VF750 disk to fit? Am I missing a step as far as adjusting the caliper or something?