[FONT="Comic Sans MS"][COLOR="#300090"]Crochet stitches have both width and height differences, so changing the hook size will affect both the width and the length of the garment. :think:
If it is a cardigan, reducing the hook size may just give you a cropped cardigan with 3/4 length sleaves and a waist line rising with the tides. So you might be able to do that if you add rows following the same pattern. But the look and feel of the fabric will change too.
A small hook may cause the fabric to be stiffer, with a tighter weave. The details of the pattern may even run together and diminish the desired visual effect.
The better method is reducing the number of stitches and/or rows. Identify where you can reduce the number of stitches without affecting the overall pattern. If the pattern repeats every 10 stitches and your guage tells you how many stitches to the inch.
So, it really does depend on the pattern you are using.