It depends on how thick the sweater is. If you make the seams too big, you may just end up with extra bulk at the sides which you may not want. Here's one method
of cutting (scary but do-able) that was used to decrease the width of a sweater. The early part of this link is about making sleeves and body shorter but if you persist, the author gets to steeking or cutting the sides. You might test this out first and see how it goes before you try it on your sweater.