Depends on the pattern and it's construction. Simple answer would be to use heavier yarn and/or larger needles, but if it's already using a worsted and size 8 or 9 you probably don't want to do that. If it's a top down raglan, then just do the increase rows until the back sts measure half the finished chest size you want and make the body and sleeves longer. If it's knit in pieces and sewn together - look at the stitch increments between the sizes and add another 'size' or two that way. And make it longer.