• Ok, think of it this way...
    If your circle measures 29" all the way around its edge...
    and your tube measures 28" all the way around its edge...
    And you want to sew the two pieces of fabric together...with their edges meeting all the way around. You would effectively be having to stretch the tube fabric all the way around (or cut notches) to make up for the missing inch in length.

    With them both being the same size, it means your outer edges match, and effectively the inner line of the seam is bunching up slightly.

    You need to half and quarter the tube, and the disc, and then as you sew edge to edge, keep tracing to make sure your marks are meeting.
    I also have my 'shape', in this case a circle on the bottom, and sew my boxing in (tube) on top.