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.
The joy I get from skiing, that's worth dying for - RIP CR Johnson