Using the Sight-Size method, the artist draws the subject in the same size as he/she sees it, whereas, the comparative method allows the artist to make the subject smaller or bigger.
Although it may be more difficult to get an accurate drawing, this method is much more flexible than the Sight-Size method. You can choose to use one of the two methods, depending on the subject you want to draw and on the situation.
When drawing still life in your own studio, the sight-size method would probably be the best choice, while you would prefer the comparative method when you draw in your sketchbook.
When drawing with the comparative method, you use some means (usually thumb and pencil) to denote an amount of visual length that corresponds to a visual distance on the subject. Then, you move your outstretched arm, with the thumb in place, to use that unit of visual distance to measure additional units of distance on the subject.
It is important to always measure from the same position and you should be doing measurements and checking your proportions through the whole process of your drawing.
Just as with the Sight-Size method, the artist also has other ways to achieve a correct drawing in the comparative method.
1. Keypoints to refer to
Are there important points in the subject, which the artist can orient himself/herself to? When drawing a model, this could be, for example, the larynx, the outer points of the clavicle, or the height of the navel.
The artist can look for certain angles on the subject and compare them with those in his drawing.
3. Horizontal/Vertical overlaps
Are certain points of the subject on a horizontal or vertical line?
Just as with the Sight-Size method, it may be useful to first mark the outermost points of the subject. This makes it possible to ensure that the drawing does not suddenly become too big or that the entire composition moves.
In general, we can say that both the Sight-Size and the Comparative Method have their right to exist. Both are tools artists use to create their artworks, and the more tools artists have, the more flexible and successful the artworks will be.