Why is the shrink space for C hard drive very small?

[Windows Central Question]

I just bought my surface book. I am trying to partition my C Hard drive. I thought that I have to shrink it first, then create a partition. However, the shrink space allowed is only around 3 GB. My disk size is 512 GB.
I defragmented (or now called optimized) the C drive and retried it, but no use. Could you please help me? I would like to create a new drive call it D, and leave C for windows only.
Thank you