Here's a size comparison chart that I did up quickly. For those who don't already know, crochet cottons are sized with the smallest numbers being the thickest threads. The pinkish yarn at the side is a worsted weight knitting yarn. It's there to help give another size reference as to how fine these threads are.
"Fine" is relative - I bet you've heard people make remarks about how they could never work with such "fine" thread, even when it's "thick" size 10! Personally, I hardly ever tat with tiny size 80, but I know that for some people it's their favorite.
By the way, sizes can vary from different thread manufacturers. A few months back I switched from hand dyeing size 10 DMC Cordonnet Special to size 10 Lizbeth. The size 10 Lizbeth is noticeably thicker, even thicker than the DMC Cebelia which is shown in the photo. But, the size 20 seemed the same to me in both brands. Currently in my dye batches I'm using sizes 10, 20, and 40 Lizbeth, and sizes 30, 50, and 80 DMC Cordonnet Special.