Only 2gb available to srink out of total 939Gb

After Windowsa 7 Disk management tool analyzes my drive, my summary screen reports
Total size before shrink = 939586 MB
Size of available shrink space = 2449 MB

I have defragmented to see if I could increase this space, but it remains the same paltry amount.

I want a data drive to hold at least, the very least, 16Gb, preferably 25 Gb. Why does it present me with such a small amount and is there any way to increase it?