Bootcamp Partition Size Issue

[Kantastic]

Hi folks, I'm trying to install Windows via Bootcamp on my Mac Mini, but run into a problem where I can't seem to divide the space equally between two partitions. 77GB is as much as I can partition for Bootcamp, which is odd since I have so much free space. I've attached a screenshot to show the issue. I've tried Googling but can't seem to find a solution. Any help is appreciated!