To enlarge a basic fitted sleeve sweater, think of it in 3 sections. Draw a rectangle that matches your shoulder to shoulder width, and is as long as you want your sweater to be. I call this section A . The center 1/3 is your average width for your neckline.
Place a ruler under your arm to get your underarm width. Measure only the straight across portion, not the curved portion on the sides. This will probably be 1 to 2 inches narrower than the total width. One half of this amount is section B and this is the amount that is bound off under the arms on your sweater design. If you are making a fitted sweater, this is the area that tapers on the sides toward the waist and then out again toward the hips. I call this section B
The third section is the space between the first two. I call this section C. Take one half os your chest body measurement plus ease and subtract from it the width of A and the width of 2 Bs, one for each side.. The remainder is the number of inches in 2 section Cs. As you widen section C, it is necessary to continue your increases until you meet section A. An average figure will only have a few increases here. A plus size will have more but they are essential to fill in this area and prevent pulling across the sleeves and chest. No changes to the sleeves are necessary with these adjustments as the number of rows doesn't change.
These are the changes necesary to widen the sweater, but if you have a large bustline, you may also need a dart along the side in order to get adequate front length. In knitting, this is done with short rows. If your sweater is shorter in center front than on the sides you need short rows. To find out how many you need, you will have to take two measurements. Take these measurements between two horizontal lines, one above the breast and one below. Measure the distance between the lines over the fullest part of the breast and also along the side. The difference between the two numbers is the amount of inches you need to add with short rows. Multiply this amount by your rows per inch, then divide that number by 2 because a short row actually needs two rows. Space the number of short rows along a line from a point about 1 inch below and 1 inch to the side of the apex of the breast.