The following describes how to add fullness (puffiness, volume) to an otherwise flat pattern.
No added fullness
See the first row of sketches. Start with three pattern pieces - A, B, and C. All horizontal sides are equal to each other. Sewing them together results in one smooth pattern.
Add equal fullness on both sides
See the second row of sketches. Suppose you want to make B equally full from top to bottom. Slash B in thin strips (as many as you like). Spread those strips apart. Then fill in the empty spaces with new fabric (the red rectangles). Those red rectangles are the added fullness.
Add fullness on one side only
See the third row of sketches. Now suppose you want to make B full only at the bottom but not at the top. Again slash B in thin strips. Rotate those strips so that they open up at the bottom but not at the top. Fill in the empty spaces with new fabric (the red triangles). Those red triangles are the added fullness.
Add unequal fullness on both sides
See the last row of sketches. Lastly, suppose you want to make B more full at the bottom than at the top. Slash B again in thin strips. Spread and rotate those strips so that they open up more at the bottom than at the top. Fill in the empty spaces with new fabric (the red trapezoids). Those red trapezoids are the added fullness.
Of course, MD does not have a scissors tool yet. So we cannot do the slashing operation here. You can do it in another application like Illustrator, or eyeball it within MD, or improvise your own way of doing it within MD.