It would be nice if I could consider myself big boned and blame my fitting problems on that. However, I have concrete proof that I am NOT big boned: namely, my shoulders. When I was the "proper" weight for my height, they were a little narrow for my size 6 blouses, but now that I am...well, larger...They are just plain tiny in comparison with my bust size.
I learned to compensate for this in my sewing, and it is a real pain! Thankfully, knitting is a lot easier to fit than tailored bust darts. There is some math involved, but it is only addition and subtraction! It certainly doesn't involve the rulers, tracing paper, and hours of flat pattern making that sewing involves for a proper fit.
To begin adjusting my knitting pattern (assuming it is a bottom up sweater), I start by knitting the entire bottom part of the sweater. OK, I should actually do the calculations before I start, but I usually don't really want to do the adjustments until I actually have to use them.
Once I reach the bind-off for the arm-holes, I am ready to make my adjustments. First, I calculate the number of stitches difference between the size I am knitting and the size I need for my shoulders (usually I just go with two sizes smaller). If the lower part of the sweater is not shaped (meaning there are no decreases from the bottom to the bind-off for the sleeve), the calculation is the number of stitches I cast-on minus the number of cast-on stitches for the smaller size (big size cast on - small size cast-on) .
Now I divide that number in half. That's it. That gives me how many extra stitches to bind off at each arm-hole (sleeve). I bind off the number of stitches that I calculated plus the number that the pattern says for new smaller size. In other words, I bind off enough to make the sweater into the smaller size and then continue knitting that smaller size.
Because of the nature of knits and my 5'4" stature, this works beautifully. It puts all of the extra room through the bust and stomach area and keeps the upper bust area narrow enough to fit my shoulders. If the sweater is shaped through the body, it is more work, but essentially the same calculation. I have to add the calculated number (big size cast on - small size cast-on) to the difference in each decrease.
Of couse, if the sweater is knit in separate pieces, I have to do the same to the back pieces. Other than that, my calculations work as well for in-the-round knits as they do for standard construction!