"I know if you unwind the bobbin it will widen the strand, but I never see them do this. Most of my thread is 6/0. For right now, I tie mostly midges. Is this thread the wrong size for midges? I am not sure which sizes of thread are bigger and which ones are smaller. Also, i'm not sure what size thread you should use for various applications. I was given dozens of spools of 6/0 thread, so that is what I have been using for everything. Is it necessary to change thread size for various applications?"
There is a wide range of thread sizes and types available these days, it's almost as confusing as walking into the potato chip aisle at the grocery store. Back in the day, you could get plain, bbq, and OOH!! at one point you could get ridged... and all of this was before anyone knew what a tortilla chip was =)
When I started tying, there was Champion Silk, 6/0 and 3/0 in black and white... period. Shortly thereafter (early/mid 70s) we saw Nymo nylon thread, and Danville's "Flymaster". Both Champion and Danville's were able to be counter-spun to flatten the thread to do what you were asking about above.... and when it came to midges, I always used to spin the thread tight to a smaller profile.
One thing that's different now is there are threads like GSP (gel spun polyester) and others that have a "core" wrapped with other fibers and these can't be counter-spun or flattened, which is why there are now so many sizes of thread from 18/0 to 3/0.
The other thing is lots of threads don't use the X/0 designations, but instead, use "denier" ratings... see more here http://www.flyanglersonline.com/flyt...s/part211.phpv
Good luck to you on this....