Ever knit a cardigan...only to find that you are not going to be able to button it because of your "pear shape"? Ahem.
Well, that has happened to me more than once, I must admit. So, I knit the size to fit my shoulders and upper body mostly...and if needs be...I add a gusset to the side seams to allow for the old caboose.
Me and another knitter were just in the new chat room, and we were consoling one another about "sizing". It can be a tough one to get right! Medium is too small...the next time, you knit a large...and it is too big...and back and forth we go!
Here are two photos of a cardigan that I knit last year. I clipped it together (thankfully) for a try-on before seaming the sides up.
So, I cast on 20 stitches for a gusset...following the seed stitch border pattern and then st st...and made the gusset the length to reach the armpit. I decreased the 20 stitchs gradually down to 1 stitch, which I cinched off. Then I seamed it to the two normal sides. (yes, that means I did double-duty in the seaming department...but at least I had a cardigan that I could actually wear!)
I think this gusset trick could work even "after the fact". Let's say your caboose has "enlarged" somewhat. You could un-seam the sides...knit and seam in a gusset at each side seam...and VOILA...the garment fits again!
I hope these two photos adequately show the gussets!