Reduce Size of Mac Partition
Open Disk Utility, and select the top level of the internal SSD/HDD. Then click Partition at the top. Select your Macintosh HD partition, then under Partition Information > Size, reduce the size of the partition. Note that you are limited by how much space is already used (this amount is listed just below the Size text field). Click Apply.
Increase Size of Boot Camp Partition
If resizing Macintosh HD is successful, your next step is to boot into Windows. Open the Start menu, then type Disk Management. Look for your C: drive. Disk Management will only allow you to extend the C: partition if the free space you just created is to the right of it. If it is listed to the left, you must use a free 3rd party utility like MiniTool Partition Wizard Free. Here’s their tutorial on the Move/Resize Partition feature.
If this small guide helps you, tweet me @scojjac.