I was under the impression that fluorocarbon lines were thinner than monofilament for the same pound test. I guess I was wrong. I compared the Berkley Vanish and the Berkley 100% diameter to the Stren mono I'm used to using, and it was smaller for the same size pound test than the Stren was, but when I compared it to the Trilene XL, then I saw that it was the exact same diameter as the XL was for the same pound test.

So now that makes me think I was wrong about fluorocarbon being able to cast better due to smaller diameter but still let me keep the same pound test as before. Cause I was gonna get the 10 pound fluorocarbon to have stronger line, but have it be an 8 pound diameter so it would cast better than 10 pound mono would.

It also sort of explains why the Berkley Trilene XL monofilament was always breaking on me in the 8 pound size when I pulled a knot tight, but the Stren never does. The 8 pound Trilene I used was .010 in diameter and the Stren is .011 for 8 pound. Trilene was thinner!!! I always make sure to wet my line before pulling tight too, so that's not it. With Stren I just don't have that problem though.

Anyway, my question is about fluorocarbon diameters.  Was I just mistaken due to only comparing it to Stren mono, or is it supposed to be generally smaller diameter than mono for the same pound test??  

Just compare the XL with the XT and you will see a diameter difference in the same pound test and the XT will have a higher true breaking strength.