I always, always always have this problem, I am a plus size woman and I thought it was because I have very large thighs but small waist and hips (in comparison). On pants I already own my solution is to take up the crotch. I sew from the top of the back seam around the buttocks curve up through the zipper. This usually works and if the slacks are still baggy, I take up a small amount on each side seam from the top to the hip line tapering into the seam toward the thigh. This usually works and the slacks look a lot better from the back!!