If the bagginess is below the seat, you can take some of it out by shortening the back crotch. To do this, rip the inseam about 6-8 inches below the crotch intersection. Place the front seam over the back the needed amount and baste. Try the pants on to see if you have taken in the right amount. If so, stitch from the crotch down the leg going far enough to make a smooth connection to the original seam. Be aware that this will also take in the thigh area too. The other way is to rip the sideseams in the baggy area and set the front seam over the back and stitch. Use the original stitching line of the front of the pants and make a new line on the back.