Resize Boot Camp partition

Though grgarside's answer was the correct & only correct answer at the time, iPartition is no longer the 'go to' app for this kind of task. It has unfortunately been left to languish & can't handle CoreStorage volumes at all.

The only replacement - & actually improvement - on iPartition is now Paragon Hard Disk Manager

It can not only resize Bootcamp partitions, it can fix them & even back up or move them to a new drive without breaking anything else in the process.

Full feature list here - https://www.paragon-software.com/home/hdm-mac/features.html

no affiliation, merely a satisfied user.

For all my partitioning, I use and recommend iPartition.

You can use iPartition on drives that contain Boot Camp volumes; in fact, you can even resize your Windows partitions, and let iPartition take care of reconfiguring the Windows boot process for you.