I'm a little confused about my disk utlliization My daisy disk shows my disk utilisation like this. I have a 128 gb disk. Why does it show as ~72gb.
The number after "Macintosh HD" and in the centre of the circle in your image is how much space on the disk is used - not the entire capacity of the disk. Clicking an area in the chart will navigate into that folder and show you how much space that folder is taking on your drive.

If you go back to the start view by clicking "Disks and Folders" you will see the actual capacity of each disk.