I've adjusted my overscan to a minimum, but even so, 1.66:1, 1.78:1 and 1.85:1 all appear the same fully filling the screen on my set. (assuming anamorphic for all three ratios.)
Well, this is just simple math:
- a 16x9 TV has an aspect ratio of 1.78:1
- a 4x3 TV has an aspect ratio of 1.33:1
So, if the aspect ratio of the movie is higher than the aspect ratio of the television, then you will have "black bars".
Of course, overscan complicates things a little. http://www.keohi.com/keohihdtv/usefulinfo/overscan.html