I know this thread is old, but i wanted to give someone that at this date visits this site a easy tutorial.
1. Download Yosemite disk utility which you can get here:
http://www.tonymacx86.com/attachments/disk-utility-7z.167117/
2. IMPORTANT: Make sure you set boot camp as your boot device, which you can do under system settings > startup disc
3. Resize mac partition to be smaller in the yosemite disk tool that you downloaded. It can be opened with a 7z unarchiver like keka. Dont try to do this with the El Capitan disk utility, because it will mess your windows, and free space up so you cant use it.
4. Once its done, boot into windows
5. Download minitool partition here:
https://www.partitionwizard.com/free-partition-manager.html
6. Expand the space on your windows in the tool. Before doing this, make sure you have done step 2, or you may corrupt windows!
7. It will restart the computer, because it needs to be done without the disc running.
8. If you did step 2, it will boot you into windows and bam! You have resized!
9. Done!
If you do the same mistake that i did the first time (not doing step 2) Then you could try to repair the disc with a bootable windows disc or usb. Hope you see this!