Do the pockets on your trousers gape and stick out like wings? Why? because the trousers are too tight even thought it may not seem so. Lets do an experiment. Put the trouser on and hook the waist but Leave the zipper open. Now pull thr pockets closed and flat. See the result, The fly pulls open.,its just too tight across the front. Solution? get larger trousers and have the waist taken in. But what if its your expensive suit trousers. Dump the suit, or alter the trouser. It can be done if there is enough outlet cloth available at the back. Or in the case of a custom made trouser there may be outlet cloth on the side seam. Check the diagram. The available cloth is at the back. but its needed at the front. The waistband is removed including the lining, but not at the fly. Next the back seam is sewn to make the trouser larger. See the line that bisects the waistband that marks the poiunt that meets the top of the side seam. The distance from that line to the arrow on the waistband is the same as the amount let out in back. Now the two arrows are matched together and the side seam advances. The whole point of this is to breng that side seam foreward in order to add more width to the front. Now we see that the seam edge on the top of the front is longer than the seam on the waistband. A dart or darts can take care of the match up of the seams. Or the top of the front can have the fullness worked worked in by basting and shrinking as would be done with sleeves fullness. If these trousers are custom made and an allowance was on the side seams then the job would be somewhat easier. Only the front and sides would be concerned, and not the back.