
Welcome to InkscapeForum!
A line drawn with the C tool should be the same size as another line drawn with the C tool, at any zoom, assuming you haven't changed anything except the zoom. But let me ask you a couple of questions.
Are you using inkscape with a graphics tablet, by any chance? If you have pressure sensitivity, that could account for different widths. Or, even if you're using it with a mouse and no tablet, the width could be different, depending in the angle of the nib, and the direction you've drawn the line. To make it the same width no matter which direction, find the Angle setting on the control bar, and set to zero

If neither of those helps....I'm not sure....maybe a sample file would help? Or maybe someone else will have an idea?