For those of you who have a flat derriere, I bring you the Flat Seat Adjustment. I cannot personally attest to having this fitting problem, but you can find an example of a baggy butt here. You will need this adjustment if there is excessive bagginess at the seat of your pants.
To fix this fitting issue, you will reduce the amount of fabric at the crotch curve, the opposite of a Full Seat Adjustment. Draw a line from the sharpest part of the curve (at center back) to the side seam. Cut TO the side seam, not THROUGH it. That way, you have created a hinge so you can bring the top of the pattern down to reduce the length of the curve at the curviest spot.
There are other techniques on the web to fix a flat seat. Ann Rowley shares a technique that involves taking fullness out vertically, as well as diagonally here. I can't really say from experience (since I don't have this fitting woe) what would work best for you. If you have tried different methods for this adjustment, I'd love to hear what your thoughts are in the comments below.