My 501's (in non-stretch fabric) enlarge one size from wearing, and shrink back from washing (40C, normal detergent, no fabric softener, no brightener, spin cyc

My 501's (in non-stretch fabric) enlarge one size from wearing, and shrink back from washing (40C, normal detergent, no fabric softener, no brightener, spin cycle and then hung to dry indoors on a line).