|On most swimsuit patterns, adjustments to the lower body are usually described as measuring from waist at back, through crotch to waist at front, divide the difference between your measurement and the pattern measurement by two, then add/subtract the difference on the lengthen/shorten line. Well, that may work for someone who is perfectly proportioned, but if the major reason for a size difference is a large/small derriere, you can end up with a suit that is too long on one side an still not long enough on the other.
I make two "big caboose" changes to a swimsuit or brief pattern. First, I make the back 1/2" longer than the front. To match the side seams, I slash the front pattern piece from the highest point of the front leg opening to the side seam, pivot down 1/2", and tape paper underneath. This may be all that is needed for an over-40 figure that is starting to "go south". The second adjustment is a slash from the back side seam at the top of the leg opening to the back seam, perpendicular to the grain line. This slash is spread 1/2" downwards on the back seam, giving me the 1" difference between my dimension and the one listed on the Kwik Sew pattern chart.
For a very flat bottom on which suits tend to bag and sag, try the second alteration, but of course overlapping instead of spreading the slash.
I recently made a Stretch And Sew swimsuit, made the same changes, and ended up with pretty much the same fit, although the leg elastic was too small for comfort. The amount of fabric at the very bottom didn't look like quite enough, so I cut a crescent shaped slash on the back leg opening over the small "bump-out", leaving just 1/8" at each end, then cut slashes in the crescent, 1/4" apart, almost to the edge of the leg opening. A piece of paper was slid underneath and the slashed crescent gently pulled outwards about 1/2", then taped in place. This crescent was about 6" long. Sorry there's no photo, but I don't have a digital camera