why? because you can take in or let out the waist, and also taper the legs.
go to a fabric store and buy a tape measure. about 2 or 3 bucks.
measure your seat then measure the trouser seat. try on the ones that pass the test.
that will save you a great deal of shopping time.
hint for athletes with heavy thighs. measure your thighs, then measure the trouser thighs.