Most better trousers can be be made larger to a limited extent. Check the extra allowance of cloth inside the trouser back. the widest part is at the top. this is used to make the waist larger. measure the width edge to edge then subtract 1/2 inch. the result is the largest amount that the waist can be increased. Go farther down that area to where the widest part of your butt is located. do the same measure. This will show the amount that the seat can be increased. Continue down the inside to the top of the inseam. usually one side of the seam is wider. measure that side and subtract 1/4 inch. thats what can be used to increase the fork of the crotch. Except for the length thats about all that can be done to increase a trouser size. It's just not enough to make a rtw trouser a full size larger. This also usually applies to m2m trousers. Custom tailors often include larger allowances. Also on light weight fabric the extra outlet on the leg seams gives more weight to the seams so as to hang better. Compare the trouser outlets below. Ready to wear and made to measure trousers are delivered with outlets as shown. Letting out can be done as shown by the dotted lines.