you would have to sew diagonally from the bottom so the bottom became thinner and not uder the armpit otherwise there would just be a gap.
i would recommend just cutting some of the ribbing off and stretching the rest out and round so its tighter and doesnt go so wide.
flip it inside out and sew a line starting at the armpit all the way down to the cuff, taking the seam in a few inches. it will look sort of like this |\ /| if that makes sense? turn it right side out and you have a skinnier hoodie
i've done it easily. just turn it inside out, sew up on both sides at desired width, and if the arm is too saggy as well, go straight up, pivot at the arm seam, then end the stitch just before the cuffs.
just make sure the hoodie is inside out, then pin and sew in a similar pattern to the red lines, you can adjust it yourself to your preferences, but it really isn't too hard.
3rd this method. Tips: turn inside out !, pin it, and if you want to sew on a line mark the line you want to sew with tailors chalk (available at fabric stores) and yard stick.
oh yeah....yard stick may not be long enough for a XXXL tall , use side of a ski with no sidecut